In my case it was major major MAJOR stress - a year-long combination of extreme changes in both my personal life, my career, my finances, my home, and also my son (who isbasically a lookalike clone of me and clings to me like a shadow) starting school - which is where I was abused even though I had good and involved parents who warned me and would have helped if I'd told them.
The united "Frankenstress" monster looked around for my weakest spot and found it and started beating me up with it - turning what had been an occasional background memory, that I'd accepted but never dealt with because I'd never associated it with pain, into a nonstop cluster of day and night flashbacks and panic.
If the CSA hadn't happened it would have been something else: a car crash or a time I got mugged. Thank God when my insurance company said "mental health emergency services" they meant it.
"Don't think it hasn't been a little slice of Heaven just because it hasn't!" --Bugs Bunny