While I might be drifting a bit far off-topic now, I think the reality is that the plot writers just wanted Garrosh in control so they could create this crisis and so they contrived a reason for it, even if it didn't make much sense to me. I think Thrall, as he was depicted in WotLK, was not so oblivious that he would have put Garrosh in charge over the established Horde leaders. I feel the story creators might have suspended disbelief a little far when they suddenly yanked us down that road in Cata.
What I would have liked to have seen is something like:
- the Cataclysm hits. The world is in chaos. Thrall has no choice but to stop it shattering. He doesn't even have time for politics and leaves the Horde leaders to work together in his absence.
- Garrosh assumes, or bullies, his way into control of the orcs, and hence Orgrimmar, which is the seat of Horde power
- as the expansion progresses, and then into MoP, he further alienates himself from the other leaders by his xenophobia and resentment at having to be tied to their decisions. His actions become increasingly unilateral and aggressive, culminating in him breaking Orgrimmar from the Horde and taking his own path. Thrall is no longer welcome in his own city and is effectively deposed, leading up to SoO.
I'm sure there are a lot of flaws with that, which a story designer would see immediately, but I think something along those lines would have been more interesting. Of course hindsight is easy too
