The only thing I can think of, OP, is the tanks are not waiting for the cast bar before they taunt.
Im not sure what cast bar youre talking about. Im talking about when he knocks back the tank he is on and is about to one shot him the other tank taunts off. What is happening is kephess is going back to the original tank after the taunt and killing him then going back to the tank that just taunted.
The tanks are being killed by that arcing slash or whatever its called, and i know the taunts are going off because im one of the tanks and its happened to both of us, when i taunt he turns to me then flickers, kills the other tank and comes back to me.
But I havent cleared this place since it was pretty new and it might just be that something has changed or im forgetting something.