Well, this is a strange one.
Rending Claws (Necro Axe #1) puts you in combat when you kill a critter (yes, I am a habitual critter-killer). No other Necro Axe skills behave this way, nor other Necro weapons do this from my experience. But, always, Rending Claws puts you in combat for some reason every single time I use it to kill a critter.