After the stunner of a trade that sent All-Star batsman Rafael Devers from the Boston Red Sox to the San Francisco Giants as part of a five-player blockbuster, many observers are no doubt left wondering why. Specifically, why would the Red Sox part with a highly productive, homegrown slugger who’s signed through 2033, in the prime of his career, and on his way to one of his best seasons? And why do it coming off a sweep of the rival New York Yankees?
Rafael Devers traded to Giants: Red Sox superstar headed to San Francisco in stunning MLB blockbuster
Dayn Perry

The short answer is that the relationship between Devers and a Red Sox front office not exactly known for smooth operations in recent years had apparently degraded beyond repair. Here’s what Peter Abraham of the Boston Globe reported regarding those club motivations:
“Per Red Sox sources: the team’s feeling was that a $313.5M contract comes with responsibilities to do what is right for the team and that Devers did not live up to those responsibilities.
“They had enough and they traded him.”
From the Red Sox’s standpoint, Devers’ “responsibilities” is seemingly code for a willingness to change positions. Leading up to and after Boston’s free-agent signing of Alex Bregman this past winter, Devers balked at the idea of moving off third base to make room for Bregman. It was, after all, the position that Devers had manned for his entire big-league career, and it’s not common for players to be “downshifted” defensively at age 28. From the club standpoint, though, Bregman was obviously the better defender, and moving Devers meant improving what was not a good infield defense overall. Eventually, Devers relented and agreed to become the Red Sox’s full-time DH. It turned out to be a role in which he thrived.
That wasn’t all, though. In early May, Red Sox first baseman Triston Casas suffered a season-ending knee injury, which left a void at the position. By all appearances, the club wanted Devers to fill that void. He had never played first base as a professional, and he’d be taking on the role mid-stream with no time to prepare or practice for it. On the other side of things, the shift to first base shouldn’t be an onerous task for a player with more than 8,000 defensive innings at third base in the majors.
Rafael Devers trade: Winners and losers, including the present-day Giants, the future Red Sox and Gerrit Cole
Mike Axisa

Soon after Casas’ injury, the manager Alex Cora downplayed the idea of moving Devers to first base. Devers, though, said club chief baseball officer Craig Breslow did indeed approach him about picking up a glove again.
“I’m not certain what (issue) he has with me,” Devers said through an interpreter of Breslow, according to the Boston Globe. “He played ball, and I would like to think that he knows that changing positions like that isn’t easy. I know I’m a ballplayer, but at the same time, they can’t expect me to play every single position out there. In (spring) training, they talked to me and basically told me to put away my glove, that I wasn’t going to play any other position but DH. Right now, I just feel like it’s not an appropriate decision by them to ask me to play another position.”
That led to a one-on-one meeting with Red Sox owner John Henry, an unusual step that seemed to put the controversy to bed. Devers overcame an early season slump to put up some of the best offensive numbers of his career, but presumably ill feelings lingered on both sides. All of that plus the fact that Devers is owed roughly $250 million through 2033 led to Sunday’s trade. In the end, the Giants stand to benefit from the latest round of Red Sox dysfunction.