Alternatively, the healer and DPS are doing it very wrong.
Fact is, it is very easy in this game to pull initial aggro. I mainly play a tank, but as a conjurer it's even easier to take and maintain aggro simply by doing what many healers do; needlesly start spamming heals as the tank hasn't even thrown out a flash yet. Then it's just a matter of continued healing to maintain that aggro, with every flash just very briefly making mobs look towards the tank.
And of course for DPS the primary issue most commonly being simply not attacking the tanks target. Either by actually single targeting a certain mob, or trying to AoE things down.