In my humble opinion, I think when frightening things happen our brain & body go into the "fight or flight" response in order to survive. Physiologically, big doses of powerful chemicals are released as part of the process.

Part of the chemical action activates a very primitive part of the brain which practically records every nuance of what happens as part of the event. That information is stored. I think it's there forever, some of us have more access than others.

I don't know if this helps any but I hope so - for the better.