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.