I'm not even sure I'm inclined to believe the claim here. You're sayin that after dealing roughly half of the monster's health, someone was STILL able to swoop in and steal aggro? Do you just have the most incompetent tank on the server or something? That'd be like me losing aggro on Leviathan's tail partway through the fight to the healers' Medica II spams or something. And before you say "provoked!" Elite Marks are immune to provoke, I have tested this personally.