It's either that the tank knows how much they can pull and expect dps the aoe the pack down at a reasonable rate, the aoe is abysmal and the pack lived for too long and is straining the tank cd's and healer mana, or the tank pulls too much.
Personally I don't expect a lot when it comes to randoms in roulette, but I usually run with a 4 man that pulls mobs appropriately and everyone is AoEing mobs well enough.