The lore on this is somewhat vague so I personally have kind of headcanoned to fill in the cracks. Fundimentally it seems to come down to a couple of things:

1. Technique: Having methods and attacks that get and draw an enemies attention and hold it or that make the tank an annoyance that the enemy has to deal with.

2. Magic: Magic is used a lot. In this case either to pull the rage of an enemy on to themselves.

3. Positioning: Its hard to attack someone off in the distance when the guy right in front of you keeps getting in your face.

Now to get a touch headcanony. Doing damage or healing doesn't automatically equal going for the healer, even in PvP. There are a lot of ways to get something's attention beyond just raw damage.

Take Flash. Flash does no damage. Its literally just a bright flash. However also image fighting when one of the guys is constantly shining a torch in your eyes. His healer buddy might need to be taken down but your going to be very sorely tempted to punch the guy in the torch in the face first just so he will stop doing it. Knocking a guy of balance, getting in the way constantly and generally being a pest that can't be ignored will make enemies want to just get you out of the way so they can focus on killing the guy at the back in the dress.

Their are other options beyond annoyance too. Magic could be use to inspire fear or hatred. Alternatively body language can do the same if done right. Some might be instinctive. Simply look at the WARs that berserk. They literally cause terror in the surrounding wildlife driving it into a frenzy.

There is also Pain, one I imagine many of the high threat attacks use, particularly with Dark Knights. You can inflict a lot of damage without causing much pain and you can also inflict a hell of a lot of Pain without doing much damage, especially if you know what your doing and you have magic at your disposal.