For me it's mostly unresolved things. When I experience things that are to me hurtful, in the moment they happen I tend to disassociate from them, due to anxiety/fear. So in the moment I am numb or anxious (or a weird combo of both) and tend to be passive, silent and to cave in, back down or retreat.

But then later when the perceived threat is over and I'm no longer in a state of fear, then my anger surfaces. Half angry at the situation or the person/people, and half angry with myself for being too fearful and dysfunctional to assertively address problems and stand up for myself.

So it's like my brain keeps replaying it and reliving over and over, wanting a better outcome. Usually by then though it's too late, it would just be ridiculous and melodramatic to return to the situation to blow up about something that happened some time ago (especially since I probably acted like it was fine when it happened).

So instead I alternate between anger, depression and numbness until it finally goes away for good (sometimes takes years).

Hopefully others will chime in with how it is like for them, because I am curious, too.