Communication and not wanting to
I made a post on Reddit mimicking similar. This is such a chaotically weird issue that seems to be one of the greatest poisons in this game's community, and I can't tell if people are trolling or dead serious.
I've played alot of MMO's and have main classed each of the trinity understanding that I am most comfortable in a dps role where I can turn my brain off, but I also can enjoy the healer role, as its a strategic joy to watch how the battle plays out and support its victory, and as a tank...I've only felt one way, I set the pace.
I'm not saying that egotistically, but doesn't that reality make this the weirdest argument because in game the way it plays out is ALWAYS the same.
DPS or Heals pulls because they want to, have beef with someone else, are impatient, or any other foolish reason, and then the conflict begins.
Why not...wait? Why not...speak up? Because they inherently WANT conflict. I have yet to see one person actually say, hey...mind if I pull a few more...they JUST DO. That's ugly, no other way to call it. It's like you feel like you're playing alone, and your "party" has to deal with it...instead of the point of being in a party and working together.
I like that someone mentioned Duty Support and how the AI operates, but I'll do you one better. In WHAT fantasy narrative can you name a healer or damage dealer that goes ahead of the meat shield to grab the enemy's attention without being a roguish character that can escape the danger without issue?
That all said...I really don't get it other than my belief that people that do that without communicating jack, are trolls. Nothing but.