2 things:
He put on a good bit of weight over the summer that seems to be slowing him down big time.
He's been playing ball almost nonstop since the middle of the summer with the Summer League and then Euro team. He's might be tiring out.
I'm just noticing now, but his walk rate was around 6-7% throughout the minors and then only 1.7% in his limited time in the majors last year. So maybe the regression goes both ways?