Its called Tag Boosting, I do it every day:

Tag Boosting


The dude you are talking about was going in and out of his group so he got all the exp in an instance (goes out of group to tag the mobs that the group killz then goes back in teh group so the instance dosn't close). He was exploiting game mechnics becasue when he left the group the instance should have closed at least for him.