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.
Apparently Traumatic™ is different from traumatic with no emphasis. I am not a neurologist but it’s my understanding that you can sit people in fMRI (or other brain activity monitoring systems) along with other monitoring systems and watch the difference between a normal memory and a flash-back. Like the Traumatic™ will function differently in ways you can measure. I learned about it from The Body Keeps the Score but I haven’t read further than that. If you have resources that aren’t too technical let me know. Some of what was in that book was pretty soft science, but the Traumatic™ memory stuff was pretty hard as far as I could tell.