it's the skirmish AI (you should've noticed it doesn't happen in the campaigns). noticeable especially on twilight flame where lots of units get stuck trying to jump over the impassable valleys. there is no cure (unless you're a wizard and want to edit the maps). upgrading helps, but the terrible programming is the issue. if you like playing twilight flame, bomb the area around the valleys to clear the units and you'll notice an immediate improvement. doesn't happen on generals because it doesn't have the extra buggy AI that they put in for zero hour.