tldr

Either the tank uses the wrong skills or the healer does.

Spamming aoe and upping hots on targets at full health will draw aggro quickly. Some tanks can hold even that. Don´t expect it.
Ofc there may be DD´s not avoiding voidzones forcing you to heal. Not quite true. I can be more benifitial for the group to let them at 30%. They will play more careful and you can still soloheal them up as the rest of the group is stable. If they die it can be even better for the group (very rare though)

That´s the reason many ppl heal with mouseover while keeping the enemies in their target. It´s easier to recognize if it is save to heal stupidity. But you´ll get a feeling for that anyway and with some equip you won´t care about nonboss aggro anymore.