It's explicitly stated in the 5.4 trailer that the horde has broken in 2.
"They are no longer part of MY horde". Come 5.4 Garrosh is no longer the Warchief of the players nor most of the player loved NPCs.
Anyway that isn't my personal issue, my issue is with the Alliance story (particularly 5.3) feeling tacked on, and lots of times unfinished or illogical.