Sano maturing at the plate.
Unfortunately, in the field, he looking like a rookie at 1B. Sure, he will get better, but we're in the playoff hunt and 1B handles the ball a lot.
This is the kind of thing that happens over 162 games, though. As long as the ugliness is temporary, you can hold your nose and get through it because Sano is crushing the ball. Unless something bad happens in the next two months, Miguel will not be manning first base in the postseason.
It's far more likely that Schoop is the odd man out and Miguel resumes duties at third, Cron stays at first, and Arraez slides to second.