Because this way, if done correctly, you get 0 stacks on the bosses.
No stacks on any of the bosses when they reach the tanks, as well as avoiding having fearfull on one of the tanks during leap, this happeneds when DPS is good so maybe not the case for everyone but we had problem with it and handled it this way.
The bosses are always going to get stacks, and they fall off as soon as the bosses are out of range of each other. So long as the Tanks are topped off by the time of the switch, and the Zorn tank doesn't get any reflected damage from Fearful, there'll be no problem. Also, the Zorn tank is ALWAYS going to get Fearful, which is the whole point of the swap in the first place.
you gotta be careful that the tank of toth is not in the 30m of zorn during the jump, to not catch the fearful.
Tank Zorn at the tree over by the Cave entrance and tank Toth just under where the Task Master leaps down. Never have to worry about fearful.