I hope to be wrong, but it is looking a bit similar to Sampras post 2002 US OPEN.
He said he wanted to rest and not play the rest of the season ( 2002 season ), but wanted to prepare hard for the Australian Open ( 2003 AusOpen). Then he said he had no time to prepare the AusOpen but that he would come back in February. Then he said he wasn't ready so he would come back for the clay season ( 2003 clay season). Then he said he didn't feel like it but that he would prepare for Wimbledon ( 2003 Wimbledon ), and then he realized not even playing Wimbledon was attractive, pleasing enough for him at that stage.
He simply realized he didn't want to play tennis anymore, and that was it.
Nadal is five years younger than Sampras (when that happened to him) and of course he has knee problems that obviously are playing a big role here, but.......I don't know, for me it is looking a bit similar and I hope to be wrong, but Nadal may be feeling that he just no longer has THE DESIRE to win (and all of what it takes).