Sometimes an enemy being defeated... explodes. Not metaphorically like in the Villainous Breakdown sense, but in a literal, physical explosion.
In the case of robots, this could be justified, as the defeat would probably cause them to short circuit and the spark to ignite their fuel but is is not limited to robots. It happens with characters who are not specifically stated to be robots, and in some cases are implied to be organic in some way or another.
Having every enemy explode upon death was more common in the 3rd and 4th generation of console gaming, when it was mainly done to avoid drawing death animations. This is also an effect of being already dead.
See also Explosive Overclocking, Chain Reaction Destruction and Load-Bearing Boss. Subtrope of Made of Explodium.
(Vegeta hits the ground, large explosion. Later, he crawls out.) Vegeta (gasping): Why did I explode?
Neon Genesis Evangelion all the way. When the Angel's cores are destroyed, they explode and leave a lake of red gunk. Specifically, this happens to the 3rd, 6th, 7th, and 10th Angels.
Actually, quite a few of the angels did not explode, including several that looked like they were going to from the way they were killed. One of the early Angels left behind a corpse when it was killed that over the next few episodes was shown being removed piece by piece because being a giant monster made it impossible to remove it whole. I guess this illustrates why this trope is useful in stories with giant monsters. Can you imagine how littered with giant bodies the cities would be if shows like Power Rangers didn't do this? although come to think of it, why didn't they just the giant robots to move the bodies?
In Higurashi no Naku Koro ni Kira's Ayakashisenshi-hen, Rika and Satoko make this happen to the Ritual Tool Devil Nail Ripper after hitting it with 07th Explosion. The resulting explosion sends Takano flying and makes her become A Twinkle in the Sky.
In Panty & Stocking with Garterbelt, every Ghost explodes when it is defeated, with a scene transition to a real world model of the Ghost being blown up. In episode 13, Garterbelt and Corset explode the same way when killed, with Garterbelt even un-exploding when he is brought back to life, complete with a reversed Death Cry Echo.
Gundam has an odd variation: In nearly every series, ships will almost always completely explode if destroyed onscreen. But at the same time, scenes of past battles will always have the wrecked hulks of ships that didn't explode when they were sunk.
Mobile Suit Gundam Wing: While UC mobile suits often don't explode when defeated, in AC, they will explode upon defeat, no matter how little damage the suit apparently took.
Return of the Jedi: The evil Emperor apparently explodes on impact after being thrown down the shaft at the climax of the film.
In the Wes Craven film Cursed, when the werewolf who infected all the other major characters dies, he inexplicably explodes in a burst of greenish energy.
Double Team: In perhaps one of the most awesome endings to an action film: The Bad Guy (played by "Mickey" Rourke) is left standing on an armed mine, with a Tiger, in the middle of a Colosseum. Just as the Tiger is about to claw him, he steps off, where the whole place explodes. Completing the total madness, Jean-Claude Van Damme, Dennis Rodman, and the actor who played Belloq outrun the ensuing fireball, hiding behind a Coke machine, while carrying a baby!
Some of the Godzilla films has the enemy monster explode after the titular monster kills them. The early films don't do this however.
Most of the monsters don't so much "explode when they die" so much as "get hit with a breath blast so ferocious it rips them into flaming debris"...except Megaguirus. She gets set aflame with an atomic blast, falls out of the sky..and then her entire body explodes when it hits the ground.
The Lord of the Rings: Sauron explodes with the force of a tactical nuke after Isildur cuts the ring from his hand in the prologue.
Before the prequel trilogy came out, the Star Wars Expanded Universe almost universally had dead Jedi fade away while dead Dark Jedi or Sith exploded violently the way the Emperor did. Later it was retconned into a power they had, a kind of Taking You with Me thing. In the Hand of Thrawn duology, Mara Jade once makes this sardonic remark when she's in need of explosives.
"Too bad we don't have a Dark Jedi handy we could kill. Remember that big blast when C'baoth died?"
Draconians in the Dragonlance trilogy have different death effects. Baaz draconians turn to stone, Kapak turn into acid, and Bozak draconians explode.
This is explained in early Kamen Rider seasons as the Monster of the Week and Mooks being equiped with self destruction mechanisms, to prevent leaving any evidence behind. However, little to no explanation is given for later productions. In their early days, SHOCKER's creations as well as their victims ended up dissolving instead of exploding. Possibly for budget reasons.
At least once, doing that didn't pay off: once in Power Rangers Dino Thunder, they blast, then turn and pose... and the monster wasn't dead, and blasted the Rangers in the back.
Also, despite this trope, every series will have the heroes think a monster dead without seeing the customary kaboom at least once. This counts as Genre Blindness bordering on Too Dumb to Live.
Sometimes, even if the monster does go boom, there's no guarantee that it's finished yet. The Hatchasaurus from the first series was one example. The Cardiotron inside it was able to reasemble the pieces of it even after it was blown to bits, and the Rangers couldn't destroy it until they destroyed that first.
Kamen Rider Kiva subverts this trope as the monsters are a race of vampires made from stained glass, called the Fangire. When they are defeated, they shatter into pieces instead of violently exploding.
In the Dragonlance setting, both Aurak and Bozak Draconians explode when they die.
Killing a Balor for good in the Abyss results in a quite dangerous explosion. In later editions, not only in the Abyss, this happened if you killed one anywhere.
Also the case with the Fiend Folio's dark stalkers and dark creepers, Mystara's huptzeens, etc.
Minions are a type of One Hitpoint Wonder enemy that typically come in large packs as meat-shields or choke-point holders, or because a Killer DM realizes the party has only single-target attackers. The typical, reasonable reaction is to run up, smash them and move on. Occasionally (especially if "ash", "flame" or "plague" is somewhere in the name) they create a burst-attack when defeated, damaging the melee fighter that killed them, and any other buddies that are close... As well as other minions, that sometimes leads to insane chain reactions.
And of course, there's the gas spore. A variety of floating fungus full of unstable gas that explodes if it receives so much as a scratch. It doesn't help that, unless looking closely, the gas spore can be easily confused with a beholder — the kind of monster you pretty much attack on sight.
The 3rd edition Spell Compendium includes the 5th-level "Death Throes" spell, which makes the caster explodes violently if he is killed. It ranks fairly high in the "utterly useless spell" category, since it destroys the body and thus makes resurrection more difficult. Although it can becomes a Game Breaker if used intelligently, for example combined with the "Magic Jar" spell....
The bosses in Metal Gear 2: Solid Snake as well. In Grey Fox's case it is possibly justified as he and Snake fight to the death in the middle of a room filled with mines. This is then retconned in Metal Gear Solid, where his intact body is outfitted with a cyborg exoesqueleton.
Red Blaster from the same game is also justified as he fights with grenades.
Justified in Metal Gear Rising: Revengeance. Upon defeat, enemy cyborgs have their memories wiped and their bodies explode. Doktor explains that this is so that the manufacturers of their cyborg bodies can keep their company secrets... secret.
All the enemies in Beyond Good & Evil. Exploding on death must be a quirk of the DomZ's physiology.
Any player in Quake III: Team Arena and Quake Live who dies while carrying the Kamikaze holdable powerup. When dying, (and if they're not gibbed) they will produce a huge explosion which shakes the arena.
Unreal Tournament: The carrier of the Vengeance Relic in matches with the Relic mutator will explode violently upon dying.
In Half-Life, any Gargantuas you encounter (the big, hulking aliens that shoot fire at Gordon) will die with a chain of explosions whenever killed.
Mega Man Classic: Just about every enemy explodes upon defeat; partly justified in that they're robots, but an explosion happens regardless of whether or not the type of weapon used would logically cause an explosion or not. (For instance, Storm Tornado in Mega Man X). The only enemies that don't explode are Proto Man, he simply stops and teleports out of the arena (though arguably he really isn't an enemy) and Dr. Wily (the machine he's piloting does though) because he falls out and surrenders, heck even Mega Man himselfexplodes into a billion energy orbs when his bar thingy with about 28 other bars...runs out. Robot Masters explode into the same orbs, which Mega Man picks up to obtain a new weapon.
Almost everything in the Mega Man Battle Network series explodes upon defeat, even the viruses that appear to be completely organic in nature. Only navis controlled by the player are exempt from this, but they'll explode just like everything else when they're fought as enemies.
The Legend of Zelda: The Wind Waker: All monsters explode into ominous (yet harmless) purple smoke. Except for the giant Armos Statues, who explode like bombs and does damage when you are too close to the explosion.
In Borderlands 2 the DLC playable character Krieg has an ability that makes EVERY enemy you kill explode EVERY time with whatever element with which you just killed them.
Everything you can pick up (including enemies) in Superman 64 explodes. Even boxes.
Many augmented soldiers in Deus Ex are programmed to violently explode upon death and can easily wound or kill the attacker if they are too close (no doubt this was intended as a feature). Some even have a spoken password that will immediately trigger the explosion, thus causing defeat instead of resulting from it.
This is justified in Iji by a self destruct mechanism to prevent either of the two alien sides from picking up the other sides tech and reverse engineering it.
In Worms, worms with fully depleted life bars take turns whipping out Plunger Detonators and going out with a One-Liner and a bang, leaving grave markers behind. The explosion can hurt nearby worms.
In RosenkreuzStilette Freudenstachel, this is what happens to Iris when Pamela deals the fatal blow to her at the end of her side-game after defeating her.
Several enemies and other bosses, especially fortress bosses, in the Rosenkreuzstilette series tend to suffer this fate upon defeat. Then again, this is a Mega Man clone, after all.
Team Fortress 2 characters who wear the Bombinomicon badge invoke this trope on their death; unless they were already gibbed by an explosion their body will explode a second later (this doesn't do any damage, though).
In "Ayla and the Mad Scientist" in the Whateley Universe, Team Kimba goes through some 'Dark Phoenix' holographic simulatins, a couple of which use Tennyo as the Dark Phoenix character. Phase uses his powers to stop her, and Tennyo explodes. Since she has anti-matter in her blood when she exerts herself, this doesn't work out well. In the simulation, it destroys the entire eastern seaboard of the United States.