Well a theory says that Garrosh's best friend/advisor Malkorok is Kil'jaeden. And that's why his personality changed so many times.
There is a tread on this at the official WoW forums.
And there is also a video about Garrosh in general here.
If you watch it you'll know exactly what I mean.
That would fit with the upcoming burning legion expansion and that's why.
Garrosh will be corrupted or by The Sha or by Demons or by The Dark Heart of Pandaria and become brainwashed by Kil'jaeden (that already is).
That's where we strike!
We will overthrown Garrosh (not kill) and he will follow Kil'jaeden with some corrupted orcs to the Twisting Nether to help him with the invasion!
Tell me your opinions on this