Assumedly, Cairne comes to Orgrimmar some indefinite time after Garrosh took over, to challenge him for the title of Warchief in a non-lethal duel. Cairne nearly wins, but then stumbles due to having been poisoned by Magatha over the last months or years (with Morrowgrain, as Malfurion was poisoned?) and runs into Garrosh's blade, mortally wounding him.
While Garrosh is stunned (and/or flees Orgrimmar) over this accident, the Grimtotem seize their chance and take over Thunder Bluff. After coming to sense again, Garrosh leads a liberation force into TB and defeats them, installing Baine as his father's successor.