Po, after landing in his hometown, now in ruins from Lord Shen's invasion, finally completely remembers why he lost his parents. Filmed from Po's perspective, his mom, pursued by his soldiers, intentionally leaves him in a pile of radish cartons so he would at least be spared and undetected, and before she lured away Shen's forces and he never saw her again. This is especially moving considering how, previously, Shen tries time and again to convince Po that his parents did not love him, just like he believes his own parents did not as well. The sweeping music, combined with the switch from 2D to 3D animation, and his mom's pained expression from giving up her baby, pulls at the heartstrings even more and making it hands down the saddest moment in a Dreamworks Animation film.
Ditto combined with Fridge Horror as you realise Po's line from the first movie about him eating while he's sad. His mom left him in a Radish crate that was full, and when he got to Mr. Ping's restaurant it was empty. On the bright side, it also makes his other line in the first movie, "I'm not hungry" all the more epic.
It's the little things that make that scene, like how gold is the color of heroism in these movies and Po's mom is literally glowing gold as she leads the wolves away. Or the fact that she is last seen with Shen himself chasing her down.
Even more sad is that it is implied that Po's Mother was killed.
Master Thundering Rhino being blasted to bits by Lord Shen's cannon.
Not to mention the followup: While Po and gang are being led to Lord Shen's palace, they come across a big groove in the ground and Master Rhino's hammer stuck upright at the end (with a large dent in it from the cannonball, no less). Whereby Wolf Boss pats it and chuckles evilly...
It's actually worse if you watch Master Rhino in the holiday and masters special's because you get to see how noble and benevolent he was before he died. The masters special even shows us that for a long time he felt as though he could never quit live up to his father's Kung-Fu legacy.
Tigress's Futile Paw Reach as Po is shot. Later, Po finding a badly injured Tigress after she took the bullet for him.
Boss Wolf's expressions as he struggles with the need to obey his master and the need to protect his pack. It's worse when you take into consideration that he had been childhood friends with Lord Shen, and then he is thoughtlessly, instantly murdered by Shen when he decides to stand up to who had once been his friend in a hopeless effort to save his kin.
The sight of all ten kung fu masters, the most powerful and noble warriors in the land, in the aftermath of the battle completely drained and utterly defeated, seemingly without a chance in the world... the shot of Shifu watching Po- either a surrogate son or a dear friend to him- looking as if he was preparing for a Suicide by Cop makes this troper's heart break.
"My son is alive."
Makes it even sadder is that for a long time Po's biological dad had believed his family was killed that night.
Tigress when the Furious Five are all in chains after seeing Po take a shot point blank from the largest cannon in Shen's arsenal. It's not hard to see that the apparent death of her closest friend has completely broken her. The only thing she can muster when Shen comes to taunt them about their defeat is a low growl and a look of pure, undiluted hatred.
Shen's normally stoic demeanor when talking about his parents breaking, if only for a moment, when the Soothsayer told him that they loved him so much that sending him away killed them. For the briefest of instants, you see a look of regret and maybe a little sorrow on Shen's face before he tries to cover it up.
Ping's freak out as he's worrying about Po will make anyone tear up. Fridge Brilliance takes it one step further, as you realize than even though Po's a giant, fully-grown panda, Mr Ping still thinks of him as a baby, like any parent would.
Mr Ping: Why'd he have to go and save China? I mean, I know why...but why? *voice breaking* He's just...one...little...panda...
Mr. Ping breaking down at the end, when he thinks Po isn't coming back, and then Po reappears.