MP,
Call them "unresolved" issues, or triggers or whatever, they can happen any time or place. One of my worst episodes happened on a quiet night at home when things were going well. As I closed the back door all I could here was rain on the leaves in the back yard. That sound took me back to nights in the mountains near Laos in 67.
Why that night, and those sounds I'd heard many times before? Hell, who knows. But, I learned that as bad as the episodes can be, they will pass. And, as you gain experience dealing with triggers you'll learn how to identify and avoid them.
I've been dealing with this shit for over forty-five years, and am back in therapy at the Vet's Center in Tacoma, WA. There are periods of time during my combat experiences that are a complete blank. Those periods may have caused triggers that my consious mind doesn't see, but are perfectly clear to my sub-consious.
Point is, the more I learn the better I can deal with the crap day by day. It never gets easier. But, you'll find better ways to deal with it. So, KEEP TAKING THOSE BABY STEPS. They'll lead you to a better place.
SD