As a tank it annoys the shit outta me.

Problem is, most people who do it are trying to force the group to go at their own speed, decide by themselves what the group can and cannot handle and usually pull a mob THEY can't handle long enough for the tank to get back aggro on it, if the tank even realizes another mob's been pulled.

In short people do it without warning. If you think the group can handle more, say so. The tank will pull more and everyone knows what to expect. This is teamwork, not you (whoever) running a dungeon with bots.

Let's just show basic respect for each other: this is OUR group, not yours. Every one of us has their role to play.

But when it happens and you (whoever) can't handle the mob: I'll just let you die. Sorry XD