I randomly stumbled upon this a few days ago and posted a thread about it on the stonehearth subreddit but I wasn’t entirely sure.
I have since tested it 2-3 more times and yes, it is for sure the skull bone pit that’s causes your footmen to go berserk and destroy nearby objects of yours and even kill each other.
When they attack your objects nearby the damage gets transferred to them instead and ends up killing them.
Footmen Fighting Each other
Footmen Dead Afterwards