Its not allways the Dps, also the tanks are prettynoften just bad.
i very often see tanks who pull, concentrate on the aggro they have on their mob marked with an 1, and in the meanwhile my aggro meter switch from green to yellow, to orange and then red, while keeping the life of the tank up.
i cant count how many times i told the tank to rotate through the mobs, after they build some aggro on their primary target.
sometimes i need to switch to stoneshielding the tank 3-4 times in a row, because they dont build aggro fast enough.
Its not allways the dps.
i for myself wait to like 70% of tank health, then start with regen. after this its cure1 all the way, untill i get an cure2 proc. since overhealing produce aggro, its to be avoided.
if an tank then loose aggro to me its ( in my eyes) his fault.
but i also had very amazing tanks, who were able to pull 2 groups at once while i had to spam cure 2 for awhile and they still didnt loose aggro.