What happened to Nadal?

The decline of Rafael Nadal has been amazing to see in the last 2 years. He was losing incredible matches to players that Djokovic, Murray and Federer beat easily. Now, he seems to be improving but we have to take into account that he's playing on his favourite surface, so is still in doubt if this "recovery" will last. What were the reasons for such a sharp decline? 

I was wondering the same question. Unbelievable, Djokovic is now far better than him.