Board Thread:WARNING ALL USERS False Info\Spoilers/@comment-24438878-20130807025507/@comment-5306038-20130807153544

Could have just repressed the memories. If an experience is too traumatic you might selectively forget details...

But I'm still thinking it was an accident. Maybe a car crash or accidental fire? In "Party Guessed", Stiles had a hallucination where his father was drunk and insulting him at Lydia's party. His hallucination father says, "Everyday I saw her lying in that hospital slowly dying..."

He goes on to say how it's Stiles fault. Since the hallucinations attacked the person on an emotional and psychological level, Stiles may have caused what happened to his mother.