I've noticed the very same thing in MTW but especially in Shogun. They also walk along your line of archers without engaging, thus exposing their flank and taking heavy casualties. The steeper the hill, the more likely this behaviour becomes.

I'm not sure why they do this but I have a theory. I believe they're trying to drive your archers into or behind your infantry, like they often do with good results on flat maps. If the archers don't retreat as quickly on steep hills as on flatlands because they know the enemy can't reach them as quickly uphill, and if the AI thinks it only has to get as close as on flatland, then the archers may not be driven back at all. If the AI tries this repeatedly we might get such erratic movements from them. This sounds plausible to me, but I'm only guessing.