While it might be the tank's fault that he couldn't hold aggro due to the amount of adds running around and the intensity of the fight. But most of the time I done it, when a healer took my aggro, is because they over-heal the MT (or OT). Common mistake for healers are that they believe spamming Cure/Physick non-stop would maintain the tank alive. While spamming it would definitely keep the tank's HP topped up, however, that is just a waste of resources and over-heal do gain aggro equally as a normal heal.
It is very much a team effort (as well as you) on how you'll perform. Make sure you talk to each other if you notice these kind of problems.
A little trick is to pop hallowed ground immediately after the adds have gathered around you in p6. This gives you a clear 10 seconds to gain threat. This 10 seconds will definitely decide the fate of your fight. Good luck.