I think you may need to look back through some of the Horde quests spread throughout Pandaria, 'cause they all contain clues about Garrosh behaviour, and one of the extremely early ones in the Jade Forest, where you fight your way to the top of a temple, with demons and orcs and goblins in it- ending in a corrupted orc filled with Sha energy, suggests that Garrosh is making an army as well as using the Sha energy himself. Consider him to be a bit like Cho'gall! We knew that he was corrupted by Sha long ago, really, it shouldn't come as a surprise to anybody that this is the final result. =)