Ah, good to know/see.
Even if we basically did nearly anything the same way you did and I still have no clue what caused the debuff... it's really good to see that it "works".
I'll have my guild/friends-OP start on the same spot (unstimed, unflaged etc.) and try to narrow it down from there.
Ensure your healers dont have any bubbles / HOTs on people and your offtanks are not guarding anyone at start. It took us a lot of wipes to understand this
. What we found happens is that a healer places a bubble on tank, tank pulls and takes damage which is absorbed by the bubble. This counts as healing, but the healer hasn't got the drouk hunter buff yet. So healer gets debuffed and dies. Same for scoundrel HOTs.
Basically, stay close to the boss at pull, and don't perform ANY offensive / defensive action till you get the drouk hunter buff.