Quick mage question since I don't see a real definitive answer on elitestjerks or wowhead through a perusal. What controls sheep breaking in PVE instances? I think spell penetration is only PVP, so is it hit chance? If I have 0% chance to miss an even level target and it's still breaking super fast it must be something else (speaking of trash in raid zones that's even level, and the packs towards the back end of VP).

I just hit 85 yesterday on my mages and am having a grand time otherwise.