I've had horrible DR and DP as a child. It runs in families with anxiety disorders and personality disorders.

No matter how many times it happened, it was always equally scary - feeling like a passive observer of a movie starring some piece of flesh and bones as the main character, feeling completely separate from that body and unable to control its decisions. The episodes usually lasted <30 minutes.

I don't know about its occurrence in psychedelics, but in my case it always occurred after periods of extreme emotions (seeing a classmate die and being aware of my own mortality, being rejected by some 'friends' in school, and a few others). The way I see it (and some neuroscientists claim), the brain shuts the perception of "self" in order to stop intense emotional pain.