We know that Deathwing is going to tear Azeroth up in a big way. Take a look at the Cataclysm trailer for how destroyed the land is going to be. Barrens ripped in half, parts of Azeroth's climate will change, and general hell will come forth unto the plentiful lands. We also know that the world event ushering in the Cataclysm is going to be big, really big. The event itself is not in the patch files yet.
Or is it?
There are invisible mobs called Shakers in the game right now. There are two types of these mobs. The first is just called [DND] Shaker that was potentially in the game as early as patch 3.2.2, based on comments left on the live version of WoWHead. There was also another mob added in patch 3.3 called [DND] Shaker - Small.
People on the PTR briefly reported seeing these mobs as targetable entities in Stormwind. They were mobs without any textures, with only their nameplate showing. You were able to damage them, but never kill them. Just like a target dummy. After a PTR patch the mobs became untargetable and entirely invisible again.
It is possible that these mobs are the hidden game mechanic causing the screen to shake. And given that they appear to have been activated with patch 3.3, it does make at least some sense that they could be tied to the beginning of the Cataclysm world event.
Of course, there is no official or unofficial word from Blizzard on this yet. Kisirani, the world designer, has been quiet as of late, and she's usually the one to let on hints about world events and other hidden things like this. We'll keep and eye out and an ear to the ground for more information, but we thought given the evidence out there now this was worth letting you all know about.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.