I'm not a neuropsychiatrist. I'm not even a psychiatrist; I'm a general practitioner. But I'll give the dumbed-down version, as I understand it.
In the presence of that kind of hormonal flood, the brain actually processes memories differently. The memory of the last time you went to Disney World is in your brain, right, but it's there in a very mundane and normal way. The memory of the situation you were in when you almost died is processed in a different way. It sits in a different part of your brain, where it's more hard-wired into that whole hormonal mix that leads to the fight or flight thing. It makes you anxious.