Sometimes prospects play better when they first make the majors. They're happier there, and not just because of the money, but with easier travel, better meals, better accommodations, and much better playing conditions, on better fields with better lighting, truer hops, easier to see pitches that are more around the plate... until someone finds a flaw in their swing, and "the book" circulates throughout the league.
Usually by the next season, we'll know if they're really ready for The Show. Boston has seen its share lately who weren't.
Meanwhile, there are plenty of guys who struggle out of the gate, like Pedroia or Casas (or Willie Mays) who find it, after adjusting.
But someone who tears it up in the minors has nothing left to prove by getting sent back down. It's usually that the parent club deems he's just not good enough.