The Biasts have a weird AI. They spawn aggroed to the cannon and are basically queued up to do an AoE attack on their target, but that requires them to be close enough, so even if you try to pull them they'll still go in to do the AoE since they're already set to do that attack. Stunning them cancels that out and they'll do their next attack, which should be on the tank if they attempted to pull them. If not they'll just go do another AoE on the cannon.
That's my experience with them anyway. When I did it I put down the pitch in their path to slow them down and did the stun shot the moment I saw the tank try and grab them, they then went straight for the tank and never got close to me. Probably more sensible to just have the tank pull them with stun.
