The Legions current invasion is Thralls fault.
He made Garrosh warchief, and should have known he wasn't right for the job. (He wasn't exactly shown as being Warchief material during Wrath)
better still he refuses to admit he might have made a mistake. - "No, you chose your own destiny" - Bitch you made the psycho into Warchief!
So yeah, Garrosh goes nuts, ends up going to AU Draenor, shit happens, now Gul'Dan is in our world and gets the legion called back to Azeroth. If Thrall didn't fuck up with his choice of Warchief, Varian would be alive, Vol'Jin would be alive, and there wouldn't be demons fucking up Azeroth.