It's very random. If I'm grouped with my guildmate and he pulls with his chanter, the mob will hit the first toon it encounters, unless everyone is invis. But when I pull with my SK, using a Terror, I don't have to worry about it smacking someone else because it comes right for the SK.
Ive always assumed it was because proximity agro is just slightly higher than my drum click debuff that I use to tag mobs with. If I pull with bellow it doesnt happen, but then bellow has its own disadvantages.