I don't know if there is a reason behind nightmares. I do know that I have them a lot along with stress dreams. I also deal with sleep paralysis and I go full dream within a dream, within another dream, and so on. When this is all mixed together for me, I'll have nightmares and paralysis at the same time, and when this happens I'll have flashbacks to go along with it. Eventually, I'll wake up but still can't move and then I'll have another nightmare and/or flashback.

When it comes to your specific nightmare, I agree with Open Eyes' interpretation. If you don't get it a lot anymore, maybe you were able to overcome a few of your traumas subconsciously.