Garrosh goes back in time to Draenor age, creating alternate timeline. In this timeline orc clans form "iron horde", their mission is to conquer azeroth of the "original" universe. This doesnt really make sense since if they build portal in this alternate universe, the portal will lead to alternate Azeroth, not the one players are in, so the original Azeroth was never in danger of orc invasion. The whole premise of expansion doesnt make sense, unless somehow the portal built in alternate universe leads to Azeroth in original universe.