http://en.wikipedia.org/wiki/World_of_W ... _Cataclysm
Story
Taken directly from the World of Warcraft: Cataclysm FAQ on the official website, the story is as follows:
"While the attention of the Horde and Alliance remained fixed upon Northrend, an ancient evil has been lying dormant within Deepholm, the domain of earth in the Elemental Plane. Hidden away in a secluded sanctuary, the corrupted Dragon Aspect Deathwing has waited, recovering from the wounds of his last battle against Azeroth, nursing his hatred for the inferior creatures that infest the surface realm...and biding his time until he can reforge the world in molten fire. Soon, Deathwing the Destroyer will return to Azeroth, and his eruption from Deepholm will sunder the world, leaving a festering wound across the continents. As the Horde and Alliance race to the epicenter of the cataclysm, the kingdoms of Azeroth will witness seismic shifts in power, the kindling of a war of the elements, and the emergence of unlikely heroes who will rise up to protect their scarred and broken world from utter devastation. The face of Azeroth is altered forever as the destruction left in Deathwing's wake reshapes the land and reveals secrets long sealed away. Players will be able to re-experience familiar zones across Kalimdor and the Eastern Kingdoms, rewrought by the cataclysm and filled with new opportunities for adventure."
In other words, "We don't want to make the world any bigger than it already is with it's ridiculous flight times and long waits for ships to dock so we decided to just add a few new textures to the original world, and it is not because we are too lazy or tired from sleeping in our swimming pools made of money."
Still sounds pretty cool though. Like they may add a heroic Deadmines and drop a crueler barb which I will never ever ever get...ever.

