Denisovich said:
Ranking the factors attributing to Nadals decline (from biggest to smallest), I would say:
1. Serve
2. Yips (esp forehand)
3. Movement
4. Lack of intensity
I'd say the biggest reason for how frequently he's losing this year is the serve (he literally can't hold serve with ease against anyone decent anymore). But in general, the biggest factor is definitely movement. I mean, Nadal's serve was garbage in 2005-2006, but he was compensating with insane movement, forehand, etc... Since his serve was never his strength, his baseline game was more than enough to make up for it against most players (obviously when you play Djokovic, Murray and those guys it's a different issue. But these days he's not even getting to them).
I say movement because it covers so many other things. The loss in movement is the main reason for his forehand being so off. You get to the ball a split second later than usual and suddenly you can't time it anymore. Movement also affects his defense, which right now honestly looks like absolutely nothing special. So basically, movement has affected Nadal's two biggest attributes: forehand and defense.
Before Nadal developed a well rounded game, he really was all about movement and forehand. He loses that and the rest will fall apart.
The serve is still mind boggling though. I mean I get that as you struggle with the rest of your game things will just go bad on every front, but it is ostensibly the shot that shouldn't be affected by all of this, so quite why and how has he been hitting it so bad all year is beyond me.