They seem to prog-attack either 1) characters whose hometown is Udgaard, or 2) evil priests. I've only seen in happen to one person and they fit both those criteria, so it's hard to know which applies.

Also they're summonable. Mentioned this to Lariya IC, and she suggested I report it. If it's not a bug and you don't want to exacerbate the problem for Udgaard residents, maybe don't publish this post.