Not entirely sure what causes this, but I seem to be proccing the 4pc T16 bonus of destruction as affliction.
Seemingly random but could be when soulshards fill up? I haven't been able to pay attention during combat, a quick screenshot I managed to take: http://i.imgur.com/cLiXvH1.jpg
Yeah, there was a thread with that question a while back. Apparently your 1 free Ember you always start with in Destro will also fill up while in Affliction (unseen, cause Affliction so no Embers) and trigger the 4 set. It happens after rezzing and by the look of your screenshot also when changing specs maybe? (you seem low on mana and in talent screen so I assume you switched spec right before this)
Not sure if this should be qualified as a bug. It's certainly odd behaviour.
I noticed this also, but after it happens the first time, it doesn't seem to proc again (shame, a free 15% crit on my dots during a fight would be nice :P)