completely motionless zombie

It can be any zombie—whether they're regular ones or elites—they simply stand still under certain conditions, though I still don't know exactly which ones. My team has two guys with shotguns, one melee fighter, and an assault rifle user. Most of them have high knockback stats. I usually use the melee hero to move around the map and manage aggro. But I'm wondering if that plays a role in the bug? Because sometimes an enemy that hasn't taken any damage just stands completely still.