Not sure if this is a bug, but it seems to be given the Defiler skill description. Per the description, bounded enemies are supposed to share only damage. But I have noticed that when you or our other summons deal an effect like Stun, that effect also gets shared across all enemies bounded by the Defiler curse.
E.g., when using Lister with 2 maxed Defilers, I am able to stun lock not just the monster being directly hit by Lister, but ALL other monsters who have been cursed by the Defilers.