2 Main reasons for loss of agro. Firstpoor timing on heals. Tank initiates contact with mob, immediatly loses agro to healer. This is going to happen with really poor timing on a heal or Regen is still active.

Timing regen to tick off prior to pull is important. BUT. Tanks, mind your own buffs and if you see regen still ticking prior to your next pull click it off. Gotta have some responsibility with it.

Second and more serious form of loss of agro is middle of the battle. This can be two reasons, one in speed runs you've got 6,9,12 mobs at one time? Hard to hold agro on them all especailly with a healer dumping every bit they can onto the tank to keep them alive. This type of loss will be a gear check for the tank, who can outthreat the other?

Dont use Medica2, really, I can heal a 15 minute speed run and never touch M2.

Ultimately it's a give an take between tank and healer. Both are to blame, both can prevent it, both need to be mindful of their threat.