Difficulty is increased because of added new mining devices.
As I know - mining devices now are produced and shipped in big quantity only by BFL.
What? Isn't BFL the one that screwed everyone over?
Nope. They were wanking for more than a year, but now everything is somehow in delivery stage. Last time I have checked - everything from year 2012 is already shipped to customers, so they have only nine months of orders to deliver

. And they constantly ship devices, all this is legitimate, nobody is screwed. Except fact that nobody who will receive now their device will make ROI. I am sad for guys who ordered 500GH/s device :/
But still, they have huuuge amount of TH/s to deliver, and they can keep difficulty increase on any level they want. This sucks.