Honestly, intrinsically random. For theme and plot significant reasons no less.
This does not make it easy to write about it convincingly, without the reader thinking it's giving the characters an easy out, or on the other hand, hosing them down unreasonably. It's not like you can roll dice to convince people that you are really generating it randomly (the "wand of wonder" -- as in, not "this is wonderful" but "I wonder what it's going to do this time"), especially when you probably don't want to generate it randomly, since it needs to serve the story.
Going to have character conscious of this and talking about it. Might help. They are aware of the nature of it.