If the raid is going smoothly, everyone is focused and the alliance is flowing towards some weak enemies ready to melt them, I can see why healers would prep their spells in anticipation. Then out of the blue all three tanks decide to take a breather for no apparent reason until the star expires and gets the ball rolling again.

It's confusing but not the end of the world, and I find it much different from sprinting ahead of the tank in a dungeon between every boss. You don't fight dungeon trash in the position it has spawned in so it's important that the tank can get initial aggro and drag them along.