Debuff limit will be changed to 16 in patch 1.7

[Kevin]

The Amazon Basin had a slightly different take on what is causing the Deep Breath.  Basically the DB comes when the overall DPS on Onyxia drops below some threshold.  When that happens is when she would do a DB on the group.  This would fit in well with what Lissa listed and possibly with Mongo Jerry's case if that group had short lapse in the amount of damage being done at an early portion of that phase of the fight.
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This is my belief from observation as well. We've only gotten a Deep Breath hit when DPS has been low be it by not having DoT's delivering damage or too many people not able to fire on her. I think our last Deep Breath was when we were expecting a phase 2 to phase 3 transition and we cleared DoT's but people also stopped the damage output as well.

It fits with the "logic" of the attack. If you aren't getting hurt as bad you have time to "breath deeply" and let loose. If you are getting hurt a lot you won't be able to get that deep breath in.

Clumping my lead to this because if she shoots a fireball into a clump what do all those people do? Stop hurting her and bandage hence dropping the DPS. That is the big danger I've seen with clumps is that she lobs a fireball at the clump and lots of people get hurt.