This has been bugging me for a while. I know every race in the Horde, aside from the orcs, and the entirety of the Alliance, have a reason to want to oust Garrosh. But the orcs don't. Everything he has done has been for them (stated in 5.1 and Heart of War), and lets be honest, Orcs have it much better under Garrosh than they do Thrall. Blizz has even gone out of their way to show that the much younger ones have all but forgot/dont care about their past with Demon Blood. Growing up in internment camps and seeing their parents weak and sick probably only inspired their hate for the Alliance.
Remember, Garrosh embodies pretty much everything an orc venerates. He is courageous, proud, quick and decisive, aggresive, and has done a lot for his people. This, along with any propaganda that the Kor'Kron spew out, would make this guy a legend in the eyes of the average orc.
So what could he possibly do to cause them to rebel? aside from a really stupid copout like, 'he went batshit insane and starts killing orcs for lulz'. The only thing that I could see happening is that the horde is going to start getting its ass handed to it by the Alliance, or Garrosh does something that causes him to lose a major battle and kill a large chunk of the orcish population im the process.
Maybe the orca wont even rebel. Maybe those with sense are going to be under lockout during the Seige.