In the video, was that tank a Paladin?
I feel fairly certain that the adds' aggro table is accessible even while they're just Quicksand. Prot Pallies do quite a bit of healing, and it's affected by their threat modifier if I'm not mistaken, so he'll be building up a lot of healing aggro as the fight goes.
If you don't have a Prot Pally then they'll make a beeline for your best healer. Because Blizz has a sense of humor. :>