When the tank/puller aggros a mob, they have hate on that mob. If other mobs link due to that action, they will initially aggro the puller/tank. However, their level of hate is at the absolute minimum that it can be. That means that any action on your part, no matter how small, on those linked mobs will make them aggo you instead. You have to wait until the tank and/or the rest of the DDs establish hate on all the mobs before you start indiscriminately AOEing.

This is why Marauder is a good tank in multi-mob situations - MRD has lots of AOE abilities that can establish enough hate so that your poison won't pull aggro in the first few moments of the fight.

In summary: Your tank doesn't know how to properly tank multiple mobs, and you aren't dealing with them in the proper manner.