Conversation so far:
Does Legion have a secret identity?
Legion is the mutant son of Professor Charles Xavier and Gabrielle Haller.
What superpowers does Legion have?
Legion has the ability to alter reality and time, but due to his multiple personalities, in practice his abilities vary.
What kind of psionic powers does Legion have?
Legion has psionic powers such as telepathic, telekinetic, pyrokinetic, among other powers.
Why does Jack Wayne control Legion's power?
David saved Moira and Wolfsbane from a fatal accident by accessing the telekinetic abilities of his Jack Wayne personality. However, this allowed Jack Wayne to take control of David's body.

Question: What was the trauma that caused this?
Self-contained rewrite: What was the caused the initial manifestation of David's mutant powers?