Yeah i think this is the problem, we have been taunting after the knock back, and not waiting for the cast.
And to the other post, we havent been putting our backs to the walls, we face an open area so we get thrown back far enough to give the other tank enough time to taunt. but i guess that shouldnt be a big deal since he stops melee to cast.
Just so you know, if your tank (tank 2) taunts too early (prior to the cast of Breath of the Masters), the other tank (tank 1) can taunt during the cast of BotM to prevent tank 2 from being hit again; if this happens tank 2 will again be laying circles. Our tanks do this intentionally on SM (we have a shadow and a vanguard). The shadow places the circles the whole time and the vanguard tanks the boss the whole time. it takes far more coordination to do intentionally, but it does happen on accident some times.