When I first saw the trailer, I didn't really get what was going on, it left me really confused. Then I watched the first panel and I got it a little better, it's not our past it's a separate past. Still didn't get how a dragon suddenly gained the power to travel to a parallel universe though. Then while reading blue posts I finally got it.
Garrosh isn't going into another universe, he's going to the past, and it's creating a splinter timeline. Still not effecting us, but the timeline is being split. What does this have to do with Deathwing you ask? Everything. This split is only possible, because the bronze dragons lost their powers. When we killed Deathwing, the Aspects 'great purpose' was fulfilled and apparently the Titans put in a remove powers clause on them. One of Nozdormu's jobs was to kill off non-alpha timelines. Basically he kept time clean, any time there was a split in the timeline, the 'dead' one was removed and it kept Azeroth safe. Safe from things like someone going back in time, creating the Iron Horde, and bringing it to attack the present.
So if we hadn't killed Deathwing, if we had just beat him but let him live, then Nozdormu might still have his powers. And if he had his powers, this off shoot timeline wouldn't be possible. There for, this entire expansion, is our fault.