Morrison, 24, was sent to the minors Aug. 13, shortly after hitting third in the batting order in a game that night. At the time, the Marlins told Morrison the demotion was for baseball reasons -- citing his ".240s" batting average as the reason for the demotion.
But hours before the game, Morrison had skipped a team meet-and-greet with season-ticket holders.
The next day, Marlins president of baseball operations Larry Beinfest told reporters Morrison needed to learn more about "being a major leaguer."
Morrison and his agent, Fred Wray -- along with the players' union -- believed the demotion was a form of discipline without just cause, so they filed the grievance Aug. 25, two days after he was recalled from Triple-A New Orleans.
"I'm doing this because I'm standing up for what's right," Morrison told ESPN.com this week. "If I thought it was because of my performance on the field, then I wouldn't be filing a grievance."
A Marlins spokesman declined comment, as did Major League Baseball. Union spokesman Greg Bouris wouldn't comment specifically on Morrison's case, but said the union encourages all of its players to report any violations of the uniform players' contract in the basic agreement.
When he was sent down, Morrison was second on the Marlins in OPS, second with 17 homers and third on the team with 60 RBIs. The move instantly created negative backlash with many fans and on Twitter, where Morrison has developed a popular and passionate following.
Morrison has been a lightning rod at times this season for Marlins management. Among the issues:
• Morrison publicly criticized San Francisco Giants general manager Brian Sabean after Sabean blasted Marlins outfielder Scott Cousins following a violent home-plate collision that ended catcher Buster Posey's season.
• Team president David Samson asked Morrison to tone down the nature of his tweets, some of which were considered to be lewd.
• After Marlins hitting coach John Mallee was fired in early June, Morrison all but blamed the firing on team owner Jeffrey Loria.
• The Palm Beach Post reported that Morrison had confronted teammate Hanley Ramirez behind closed doors about his late arrivals.
Then came the Aug. 13 meet-and greet with season-ticket holders, which Morrison skipped after union rep Wes Helms told Morrison he could. After the game that night, Helms was released and Morrison demoted.
Three weeks ago in New York, Loria staunchly defended the move, insisting it was a "baseball decision."
Loria said he wasn't mad about Morrison skipping the charity event.
"He got bad advice -- period," Loria said.
Loria said he has no doubt that Morrison's comments about Mallee's dismissal were about him, but he wasn't angry with Morrison over that, either. Loria insists his relationship with Morrison has not changed, saying that it's "great."
The grievance process could take up to a year and the case would end up in front of an arbitrator, unless the two sides decide to settle.
Should the Marlins lose in arbitration, it would mean not much more than having to repay the lost wages, plus any interest.
But it also means that one of their young, rising players has alleged the team was breaking the collective bargaining agreement -- a serious charge.
Morrison said the money is not what the grievance is about.
"I'm doing it just not for myself but for other players," Morrison said. "I didn't want to file a grievance, but it happened. I want to put it in the past and move on. But you've got to take a stand somewhere and based on the guidance of my agent and the players' association, I filed this grievance. Now I just want to move on and play baseball."
Amy K. Nelson is a staff writer for ESPN.com. She can be reached at: email@example.com or at Twitter.com/amyknelson.