By the hands of a Hellscream
Here are the basics -- following the Siege of Orgrimmar and our defeat of Garrosh Hellscream (so far the dude keeps dropping axes every time I zone in) he is taken prisoner and held in order to try him for his crimes. Garrosh, of course, doesn't seem himself as a criminal at all -- he intended to make a new, better land for the orcish people as a true orcish warchief should, and in the end he was balked by these strange aliens Thrall allowed into the Horde, which to his mind was always an orcish institution (and to be fair, he's got a point there) - so Garrosh, rather than accepting any punishments that might have been handed down to him, breaks free and escapes. But how? With the aid of an as-yet canonically unnamed (but if you ran the Timeless Isle, you probably know) accomplice. And with this accomplice, Garrosh does something most of us can only dream of.
The central event of Garrosh's life was the foundation of the Old Horde, when his father Grom Hellscream doomed his people to slavery by being the first to drink of the demonic blood of Mannoroth. The shame he felt before learning of Grom's redemption drove his life for years, and the pride that swelled his chest at learning of it drove him to the heights of arrogance he displayed as Warchief. The creation of the True Horde was born out of a desire to be as great a hero for the orcish people as Grom was - to save them, and give them a fertile new world to replace the shattered Outland they had come to inherit. The heroes of the Old Horde were Garrosh's heroes, but that one, terrible mistake, the drinking of the demon's blood, the blood curse was the first step on the path that ruined everything and in Garrosh's mind led him to that cell, betrayed by Thrall and the others who couldn't see the true path for the orcish people - that they lived even before the formation of the Horde as peerless warriors and conquerers of their harsh, unforgiving world. That conquest was in their nature - that with or without Gul'dan, the Horde was an inevitability, and that orcs could, nay, must rule over all.
Garrosh's accomplice gave him the chance most of us never get. He somehow took Garrosh back in time, to the time before Grom drank the blood - and he stopped it from happening. Gul'dan's influence was usurped, and a new path to power was laid out before the orcs. With knowledge brought back from the future, the orcs of this new Draenor (spared the corruption of the warlock magics and fel blood curse) are what they always were - savage, violent, aggressive conquerers of all that stands before them, and also with new technologies and massive foundries to use in building an army of conquest. This Iron Horde rises in place of the one Gul'dan would have created, and marches forth to conquer a Draenor forever severed from history as we know it. Worse, somehow this force creates their own Dark Portal, using the secrets of Garrosh's accomplice to breach not only space, but time, and begins an invasion of our world. Heroes must rise to push back this invasion as the Dark Portal goes red, then travel to this new Draenor with its new destiny to help decide what that destiny ultimately is.
Along the way we will fight the various Warlords of the Iron Horde - familiar figures of legend to many of us, altered forever by this change. For the most part uncorrupted by demonic power (save, of course, for Gul'dan the treacherous former apprentice of Ner'zhul, who still seeks to deliver the orcs into the waiting arms of Kil'Jaeden) figures like Blackhand, Kargath Bladefist, Durotan, Kilrogg Deadeye, Ner'zhul and Grommash Hellscream ride as brown skinned orcs dedicated to conquest, not in the name of demonic masters, but for themselves. In essence, Garrosh's actions have split this world and its history off from the Draenor that became Outland - it's a new world, and we're not going there to fix history or change it, we're going there to prevent the Iron Horde from conquering our world right now.
The future of this Draenor is definitely not set, and we have no idea what might happen to it, but that's why we're going, to make that decision.