Rebelling units.
Rebelling units attack other units and core. Attacked units do not respond in any way and get angry themselves. Eventually they also rebel which keeps going on until all creatures rebel and destroy the core.
Suggestion 1 : Rebel units change their faction, thus become targetable by your creatures.
Suggestion 2 : Rebel units try to leave your dungeon through portal, if they can't, they start slowly lose health while keep attacking everything and everyone. Eventually they just die out of ''hunger''.
Rebelling units becoming angry and attacking your other minions and your core is intended behaviour, however attacked units should fight back and kill the rebelling unit by defending themselves as per your suggestion #1.
Can you definitely confirm that your (still loyal) minions didn't fight back?