Stun bubble over all mobs in an area?

Is this a bug or a skill? Mobs in a whole area including bosses having the lil stun bubble above their head. I did the act 1 boss and was able to stunlock him. Which don't get me wrong was cool but if it's a skill in my party I'd like to have it myself. My party members weren't the most talkative as far as if it was from them. This lasted into act 2 and then when we split up it was gone.