The brain is lazy.

As a rule of thumb, the brain is better at storing information that is more attached to strong emotions, or anything that you repeat often.

When a traumatic event happens, the brain will try to attach it to one, maybe two things that are easy to spot and remember (say, a man in a black hoodie wearing flip flops), so that when those specific triggers happen to be around again (black hoodie, flip flop walking sound), there is enough information for the rest of the body to react as soon as possible.

This has a lot to do with survival. Think like a caveman. You hear a growl, you turn back and see a tiger or jaguar, readying a pounce. If you survive, the next time you hear a similar enough growl, you won't even need to look back, it'll be enough to trigger your fight-or-flight response and get you moving earlier. You don't need to remember what kind of predator it was, only that it was dangerous.