What do the Skull and Crossbones icons over the heads mean?

We have run across this skull and crossbones icon over a few enemies in Hell difficulty and we don't really know what it is. It doesn't appear to be consistent with effect names on elites. I did notice my character had a debuff in the screenshot (didn't notice it in game) and also has the skull and crossbones over his head. So what is this thing and what effects does it have on the monsters and the players?

I asked around and it seems that it is indeed the enrage timer that is activated when you fight monsters for too long.

If you fight a boss or champ pack for too long they hit a soft enrage where you take damage every second for standing near them

I haven't ever seen it in Nightmare, so it looks like the debuff can occur in Hell or Inferno.

This is the enrage of the mob. After a while it will enrage and do dmg to all around it. Only occurs in hell and inferno mode.

We were able to reset it by leaving the dungeon for a while (cannot say a clear number).