They are taking good camps and stealing pulls that path within X radius of them. That is very disruptive. I've had a hilarious run-in with bots in Empyr: when I pulled ph's for the named our group was hunting, an SK would run from over a hill, aggro my pulls and run away with them, without saying a word! Turned out there was a bot group on the other side of the hill, and our pulls were within their 'radius' even though they couldn't possibly see them. If you have enough DPS, you can even use this to your advantage: let the bots tank and debuff mobs for you, the xp and loot will be yours as long as you outdps them.