It's hardly speculation, it's pretty obvious this was planned long ago. Murray did the same thing to eff up the USO draw a few years back.
Murray was suffering from arthritis and did a similar thing at the AO the next year. he withdrew a little earlier, but it was still the same from his perspective, he was trying to get fit, but failed at the final tests. a few days later he cried in front of a camera and announced his hip surgery.
many players regularly withdraw at the last minute. their exact conditions are and will remain unclear (due to medical confidentiality).
and is it so different if a player already knows that he will have to pull out after 2 or 3 matches?
Nadal had to check his body after that final. he is 33 years old and realized that he would not be in a shape to make Cincy semis or finals without risking his health.
a tired player giving 100% will inevitably lead to injury and he's veteran enough to know it.
edit:
even if he kinda announced it earlier (like The Blond Blur says) he maybe had second thoughts after the SF walkover, who knows.
even a Masters semifinal is worth as many points as a Slam QF.
these are points Nadal could really need ...and all the lost/next gen guys are useless and couldn't stop him, right?
heck, he even got the easier draw, with Fedovic in the opposite half.
why the feck did he withdraw?
he could have tanked in the SF to preserve his H2H with Fedovic.
is he not even able to do that?