This is one of those chicken-or-the-egg questions that I really don't know the answer to, but I'll give it a shot even though for me, psychosis only happens during a manic episode.

I really think the depression causes the psychosis, but can't say that it doesn't work the other way. Psychosis can be frightening, which causes anxiety and stress, and those feelings in turn can cause depression. That's the only thing that makes sense to me, anyway. I hope some other folks can weigh in here because you pose an interesting question, and others whose experiences parallel yours more closely can tell you better than I can.