Planar invasion groups use the "follow the leader" model to control their behavior: These groups consist of 1 stronger leader and several weaker followers. The leader guides the whole group: Wherever he goes, the others follow. Any enemy attacked by the leader is attacked by the followers as well. And if the leader is attacked, this causes all followers to fight the aggressor as well.
So if you want to battle such an invasion group alone, you usually start doing the follwers first one by one, until only the leader is left. This does not pull the rest of the group and makes the fight a lot easier.
But you will encounter an error, if you're fighting a follower while the rest of the group (including the leader) is involved in another fight (i.e. a npc camp guard). If the target of the rest of the invasion group is killed, they reset. The leader seems to send kind of a reset message to the entire group at that moment, causing any member to get full life again and clearing their targets. This causes the follower you're fighting to reset as well: In the middle of the fight his life is suddenly fully restored and he is losing you as a target for a moment.
Pretty nasty if it's a tight fight, because the invadors are elites of about your level.
The error is that an individual battle of any follower that is completely independant from the rest of the group including the leader has to overwrite any commands (such as resets) from the leader, or cause these commands to be ignored. That does not happen.