This myth that Garrosh was only ousted because "he went against other Horde members" is just that. A myth. Prior to Theramore, Garrosh's underlings were already disobeying him right and left. Defying direct orders, leveraging loyalty to force him to do things (in violation of the Blood Oath), threatening to assassinate him, denying his legitimacy, etc.
When they knew Theramore was being attacked (but weren't aware of the mana bomb), he was betrayed outright by Baine, and there was a subversive, secret meeting held against him. Not to mention, more people disobeying direct orders.
After Theramore, we see more leveraging of loyalty to force policy change, another secret meeting in which participants openly fantasize about his ousting, and so on.
By the time Garrosh started clamping down on dissent in full, much of the Horde was in treasonous territory. By the time Garrosh formed the "True Horde" as a distinct political entity, much of the Horde had already been in open rebellion.
The fact that Thrall is even a viable candidate for Warchief at this point is disgusting on a few levels. They really should've stuck with Garrosh. The Horde's been an absolute mess ever since.
- - - Updated - - -
Of course the people supporting Baine, the traitor and Alliance collaborator, for Warchief are Alliance. Of course.