When adds spawn in, their aggro takes on what the boss has aggro on. Usually, the tank will have the default hate, but if you heal the tank, the aggro quickly switches to you. It's the tank's job to yank the hate off you and if you see adds spawn in, hold off on the heals (unless your tank si dying, then heal anyway) until the tank can pull hate on them.

If you get hate, drag the adds towards the tank. It's the tank's job to hit them hard enough until they become the new target.