Bismarck Ex bug?

So, every time I've done Bismarck EX, there's usually three rounds of the reflect debuff, with the last having about 20-25 seconds to finish off the snakes. Today I was in a group and we kept downing one snake at around the same time the third debuff came up and within 3-4 seconds Bis would come up and wipe us. Is this a bug or something intentional?

Having done it a few times, my suspicion is that the death of one of the snakes triggers an invisible timer that forces the other one to be killed in X amount of time (since DPS is essentially doubling on that one target) or else it will vanish and cause a wipe.