Headhunter Heavy belt gives buffs to NPC's and Map bosses.

I am not 100% sure that Headhunter is at fault, but it started happening right when i started using the belt and the effect is kinda connected to what the belt gives to you. One other thing i changes is a new passive from Atlas Tree that gives rare monsters extra buffs.

All of the NPC's i meet on maps now have several buffs that Rare monsters have.
Map bosses get buffs too. And depending on a buff, that can be devastating. Imagine them getting Haste or Proximal Tangebility.

The atlas passive gives uniques 1-2 rare modifiers, not a bug likely.