It’s a thing. It’s because Traumatic™ memories are stored differently in your brain than normal bad memories. Essentially the part of your brain primarily responsible for digging up memories doesn’t have the connections it world normally use to call up the memory, but the connections within the sensations and experiences of the memory still exist. That’s why a person can “unlock” these memories.

You have to be super careful trying to dig these things out though, because it is absolutely possible to accidentally lead a person into false memories.