I have had this happen to me in mid-combat with my summons as well. It will fight, then sometimes for no ascertainable reason it will simply stop (I have noticed this is usually in the middle of a mob attack). I have had success in getting it to follow me, then sometimes it will launch an attack of opportunity on a pursuing enemy, which seems to snap it out of its trance. Other times, it has sometimes snapped out of it when I launch my own attack. It is an annoying thing that I can't figure out what is causing it -- the only thing I can guess at is whatever enemy it has currently targeted has moved out of reach, and the AI is too stupid to selected a new target.