Cloud of Darkness OSKO DPS question

Every now and then in the final fight, I'll get OSKO'd for no apparent reason. The only thing I see is a "bleed debuff" and then it's lights out. As a caster, the only thing I can think of is that this is a tank mechanic (one which I inherit if the main tank goes down and I happen to be next in line for enmity). I've looked at guides on the web and haven't really found anything that addresses it. Would like to know if it's avoidable so I don't fall to it in the future. It's so random and kind of embarrassing when I've been cruising thru everything else in the instance.

Comes from her auto attack, so you had aggro. Either the tank died or wasn't tanking hard enough.

Gotcha... I'll just pop quelling strikes whenever it's off CD just to be safe. Thanks for clarifying that. I didn't have any vulnerability stacks so I was at a loss as to what could have been offing me so quickly.