I've noticed something strange in the random maps I play; some of the AI lords, for whatever reason, refuse to do anything. In a recent game, I was well over a hundred turns in when I bumped into a lord who had not conquered any provinces, was using a militia guard in his capital, and did not appear to have any heroes at all. Some of the lords do just fine, but there's always a handful that conquer a province or two and then sit around until someone else conquers them. Any idea what could cause this?

I encountered it as well.
I don't think he refused to do anything, but FAILED to do anything.
My guess is that he had his hero die too many times and his economy broke.
Possibly it doesn't have enough crystals to resurrect any heroes.