In any story in which a character changes form, regardless of whether they become [[AttackOfTheFiftyFootWhatever a giant]], an IncredibleShrinkingMan, or [[GenderBender even the opposite sex]], this is usually explained using of the following reasons:

[[index]]
* They JustWokeUpThatWay.
* A MadScientist
** An EvilutionaryBiologist experimented on them.
** They were shot with a TransformationRay or ShrinkRay.
** They were PlayingWithSyringes.
** They were injected with a SuperSerum.
*** The [[PsychoSerum Super Serum went wrong]].
** They were [[GeneticEngineeringIsTheNewNuke genetically modified.]]
*** Which gave them an UnstableGeneticCode.
* They were involved in a nasty TeleporterAccident.
** They were accidentally [[FusionDance fused together]] with another creature.
* They were accidentally exposed to a [[MutagenicGoo strange substance]].
** They ate the MutagenicFood.
*** They ate the new and improved crunchtastic [[CerealInducedSuperpowers Yummypops]].
** [[ILoveNuclearPower They were exposed to strange radiation]] or nuclear waste.
** They got too close to the GreenRocks.
* They were in [[FisherKingdom a strange place that caused transformations]].
** They took a dip in the FountainOfYouth.
** They fell into the [[SwampsAreEvil cursed swamp]].
** Their reality got [[RealityWarper warped or re-written]].
** They got hit by the WorldWreckingWave.
* They have [[BecauseDestinySaysSo always been destined to transform]] at a [[DangerousSixteenthBirthday certain point in their lives]].
** They're a HalfHumanHybrid
*** It's a natural process of {{Metamorphosis}} for them.
** They have a number of "natural" genetic mutations that don't manifest until [[PubertySuperpower puberty]].
** They inherited a family {{curse}}.
** They're "evolving" [[EvolutionaryLevels into a superior lifeform]].
** They were [[SuperpowerfulGenetics carrying certain genes]] that [[DieOrFly got "activated" somehow]].
* [[ViralTransformation Transformation is contagious]]
** [[TheVirus It's a virus]], and the character was infected (possibly through biting).
** They were splashed with a monster's [[SuperhumanTransfusion bodily fluids.]]
* They became the [[HeroicHost host]] to an alien parasite.
** They received a FaceFullOfAlienWingWong.
** They were [[TransformationOfThePossessed by something that wanted them to transform]].
* They received the form from somebody else.
** They were [[SuperEmpowering given]] the form as a gift.
** They earned their new form by WorkingForABodyUpgrade.
** They were given a SuperhumanTransfusion.
** A dying character [[ImDyingPleaseTakeMyMacGuffin gave them the power]].
** They were given a [[PowersAsPrograms copy of the form]] from another character.
** They gained the form by [[MegaManning fighting someone with it]] ([[DittoFighter although the form may not last]]).
*** They [[YouWillBeAssimilated assimilated]] or [[YouAreWhoYouEat ate]] a foe who had the form.
*** They became subject to YouKillItYouBoughtIt.
** They gained the form [[AdaptiveAbility as a reaction to being attacked]].
* They were on [[FirstEpisodeResurrection the brink of death]] and [[EmergencyTransformation it was the only way to save them]].
** [[WeCanRebuildHim They were "rebuilt" with cybernetic parts]]
* They [[FreakyFridayFlip switched bodies with someone else]]
* They triggered a TransformationTrinket.
** They [[BecomingTheCostume wore a garment that transformed them]]
** A ClingyMacGuffin got stuck to them.
* They [[BeCarefulWhatYouWishFor made the wrong wish]] to a LiteralGenie.
* They [[BalefulPolymorph pissed off a witch]].
** They [[CosmicPlaything pissed off the Gods]].
** They pissed off ''[[AuthorPowers the author]]''.
*** The author [[AuthorAppeal is into that sort of thing]].
* They were a complete jerk [[KarmicTransformation and they deserved it.]]
** They were such a jerk [[EvilMakesYouMonstrous that they started transforming all on their own]].
* It was a BargainWithHeaven.
** Or a DealWithTheDevil.
* AWizardDidIt
* They were [[LightningCanDoAnything struck by lightning]].
* They [[{{Transhuman}} wanted to change]].
* They only [[{{DelusionsofDoghood}} think they transformed]]
* [[OneWingedAngel A videogame hero has just slain their usual form]].
[[/index]]

It's also worth noting that a lot of these are also what might give a character [[HowToGiveACharacterSuperpowers Superpowers]], however, gaining a superpower tends to be more of a plot focus, whereas in a transformation story, the focus is more on what happens during or after the change rather than what caused it.