Resist?

Fully booked ?

I would guess Resurgent mastery on Skullcrown. The check for this mastery is done after the damage is dealt, so it can remove debuffs immediately from the same attack initally placing the debuff. I had the same problem quite often with Ronda vs. Mithrala.

I think, Resurgent is way to overpowered right now in PvP, and it's only a tier 3 mastery.