I've noticed this too with my wife, some mobs "link" (term used for a mob being engaged in combat to agro another mob along with it) more when teamed up.
It seems this is intended, as with if you are higher lvl compared to a mob, they will link with a greater number.

You should be able to see, when targeting, the "link" lines extending from your primary target to any targets around it that will link.
Just watch for those lines when you select a target, and if it connects with a 17... maybe move a little further away and go onto something else.