Tank is probably going slow or it was accidentally pulled. Or healers + dps are bored and want more.
Most runs i've been in, the tank just goes full blown speedrun and pulls everything regardless.

Never really understood the speedrun mentality, but as a main heal with zero ambition to pull on my own (on purpose, yeah it happend a few times with accidental pulls), i just accepted it and went on.

I am 100% certain, if there are no walls, tanks would pull the entire dungeon + bosses.