Zeds dying automatically after falling from ceiling vent

I tried to take a screenshot but I was hit by biled and swarmed/killed from behind, but it was in the paris level, in the second floor of the building with all the office chairs in it and the open window overlooking the staircase. Some stalkers spawned and died automatically next to me, I was playing solo and they were the first things that spawned and I hadn't even fired a shot or thrown a grenade yet. They just died and were hanging from the ceiling comically, one arm stuck to the vent. I was on suicidal difficulty. Not sure if that makes a difference or not but I know things spawn differently/act differently, so I figure it might be worth mentioning.

Zeds dying automatically after falling from ceiling vent