I've been thinking about a few things regarding it...first, it appeared out of nowhere, people didn't know about it, then it was buried by the Dark Below nonsense which has been proven as a fake...the Skysunder is still unproven and after seeing the conclusion of 5.4...I've been thinking that it's true.
Just get this...Garrosh wasn't killed on the spot, he was forced to have a trial. You could practically see the hate on his face when it was mentioned. He's probably very pissed at everyone for bringing him down and he wants a way out of it to get revenge on everyone...what's the one thing that comes into his mind now his mighty sha energy has failed?
Grom. Demon Blood. You get where this is going? Garrosh, furious and angry, takes the same pact that his father did, breaks free, and through this new link, the Legion begin their attack on Azeroth.
Pre-expansion event patch 5.5, The Skysunder.