I have had this happen without any of my units dying, or even taking any damage.its an odd one. Sometimes i will move a unit, make an attack, and then that's it, i have to auto to end it. very strange.
It has also happened to me that all my units have taken their turns, and then i can not skip to the next turn, have to auto to end the fight.
maybe it could be related to moving the units in a different order than the game wants you to use them (shrug a guess)