Transformation Causes

In any story in which a character changes form, regardless of whether they become a giant, an Incredible Shrinking Man, or even the opposite sex, this is usually explained using of the following reasons:

• They Just Woke Up That Way.
• A Mad Scientist
  • An Evilutionary Biologist experimented on them.
  • They were shot with a Transformation Ray or Shrink Ray.
  • They were Playing with Syringes.
  • They were injected with a Super Serum.
    • The Super Serum went wrong.
  • They were genetically modified.
    • Which gave them an Unstable Genetic Code.
• They were involved in a nasty Teleporter Accident.
  • They were accidentally fused together with another creature.
• They were accidentally exposed to a strange substance.
  • They ate the Mutagenic Food.
    • They ate the new and improved crunchtastic Yummypops.
  • They were exposed to strange radiation or nuclear waste.
  • They got too close to the Green Rocks.
• They were in a strange place that caused transformations.
  • They took a dip in the Fountain of Youth.
  • They fell into the cursed swamp.
  • Their reality got warped or re-written.
  • They got hit by the World-Wrecking Wave.
• They have always been destined to transform at a certain point in their lives.
  • They're a Half-Human Hybrid
    • It's a natural process of Metamorphosis for them.
  • They have a number of "natural" genetic mutations that don't manifest until puberty.
  • They inherited a family Curse.
  • They're "evolving" into a superior lifeform.
  • They were carrying certain genes that got "activated" somehow.
• Transformation is contagious
  • It's a virus, and the character was infected (possibly through biting).
  • They were splashed with a monster's bodily fluids.
• They became the host to an alien parasite.
  • They received a Face Full of Alien Wingwong.
  • They were by something that wanted them to transform.
• They received the form from somebody else.
  • They were given the form as a gift.
  • They earned their new form by Working for a Body Upgrade.
  • They were given a Superhuman Transfusion.
  • A dying character gave them the power.
  • They were given a copy of the form from another character.
  • They gained the form by fighting someone with it (although the form may not last).
    • They assimilated or ate a foe who had the form.
    • They became subject to You Kill It, You Bought It.
  • They gained the form as a reaction to being attacked.
• They were on the brink of death and it was the only way to save them.
  • They were "rebuilt" with cybernetic parts
• They switched bodies with someone else
• They triggered a Transformation Trinket.
  • They wore a garment that transformed them
  • A Clingy MacGuffin got stuck to them.
• They made the wrong wish to a Literal Genie.
• They pissed off a witch.
  • They pissed off the Gods.
  • They pissed off the author.
    • The author is into that sort of thing.
• They were a complete jerk and they deserved it.
  • They were such a jerk that they started transforming all on their own.
• It was a Bargain with Heaven.
  • Or a Deal with the Devil.
• A Wizard Did It
• They were struck by lightning.
• They wanted to change.
• They only think they transformed
• A videogame hero has just slain their usual form.

It's also worth noting that a lot of these are also what might give a character 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.