The Horde do, plan and simple.
They are fighting to keep their faction together, and by binding together to destroy garrosh, they are working to stop him from bringing division to them all. I don't buy all this theramore was bombed so the alliance deserve it more garbage. It was retaliation for camp taurajo an eye for an eye, and if anything the alliance have no need to stop garrosh because they must know by now, by Garrosh's actions he is destroying the horde from the inside out, because a faction divided is much weaker then a faction united. Garrosh's actions are "yes" for the destruction of the alliance, but by doing it in the manner is going about it, he is also destroying the horde.