My Theory, just as the Old Gods were the puppeteers to a power greater than there own (Deathwing), the Burning Legion is also still using Sargeras as a puppet ruler.
Further more they are known for turning nation against nation, and I can't help but think after completing "Operation: Shieldwall" and "The Dominance Offensive" that the Sunreavers were set up and that both sides KNEW Dalaran "City of Peace" would take a side.
Aethas wasn't in on this plan and clearly states in a Sha induced Rage that Garrosh will bring the horde down (Why would he then risk losing his home in Dalaran to help him.) Fanlyr Silverthorn who gets you into the city does a shit job of covering your tracks as horde. Jaina seems "different" as well. It all points to one thing.
Naz'riem Agents manipulating both Alliance and Horde into War with one another.
The Question is WHO is the Psyl.. I mean Dreadlord agent?
Speculation based on: http://www.wowwiki.com/Sargeras