I think most people have seen that if you leave Uhai alive with low HP in Chapter 18/19 (depending on what mode you are playing in), he will try and run away from your characters toward the forts. Most of the time, blocking off his escape route makes him attack you as you would want. However, I have noticed that occasionally he will instead respond to such encroachment by just skipping his turn and staying in his starting spot.
From testing, it seems to me like the times when this occurs are related to whether or not the nomads above and to the left of him have moved, but it would be great if someone who has extensive knowledge about the AI (Gryz?) could confirm this for me.