Nadal has always been prone to hitting short for a number of reasons. As a defensive player, he is hitting off the back foot more often than Federer Djokovic etc.. who are stepping in and dictating. (obviously harder to hit deep off back foot)Taking 12 extra steps sometimes, to run around his backhand means he will arrive late sometimes to hit deep. Leaving a big gap on his forehand side also means he will arrive late for those who are able to expose it. The biggest contributing factor i reckon is confidence. He is definately more confident this year and has had to earn it.