Originally posted by Weaver:

If they can clearly path to you they will focus you. Otherwise they will try to destroy objects between you and them or destroy objects if they can't see you.
They have been tweaking the AI lately to try to find a balance in things.
Note that even other mobs can block pathing to you and so mobs behind them may then be diverted elsewhere. So choke points at times can work against you.

ok. My fort was built with a moat so nothing could get to me and most mobs just throw rocks at my walls. the two that started attacking my stairs were the shaman and big club guy