A source told ESPN's Jeff Goodman that Frazier was disgruntled with his role on the team, but an academic scandal apparently also contributed.
Frazier was at the forefront of the NCAA's academic fraud investigation that resulted in SMU being hit with a one-year postseason ban and coach Larry Brown being given a nine-game suspension and two-year show-cause penalty.
The NCAA's investigation found that after Frazier was enrolled in an online course to meet NCAA initial eligibility standards and be admitted to the university, a former administrative assistant obtained the student's username and password and completed all of his coursework.
"I didn't even know what was going on," Frazier told ESPN at the time. "I didn't know she was doing that class for me. I wasn't aware of that. I know it looks that way on the outside looking in, but I didn't know."
Royce Johnson, Frazier's former high school coach at Kimball in Dallas, said that the scandal was a major contributor in the guard's walking away.
"He's been having a hard time dealing with the whole situation," Johnson said, according to the Dallas Morning News. "I just think he hit a boiling point.
"He's fine on grades. He and Larry didn't get into it. When they came down with the suspension, Keith felt like the city and everybody was pointing their finger at him as the one to blame."
Frazier, a former McDonald's All-American, was averaging 11.9 points and 4.4 rebounds in 10 appearances (eight starts) for No. 15 SMU (13-0). He was unexpectedly absent during the team's win Saturday over South Florida.
Brown said Frazier told him he was leaving the team to deal with a personal issue. Brown said he had talked to Frazier and his mother and wasn't sure whether or when Frazier would return to the team, but a source told ESPN.com that he does not plan to return.
"We're going to support him," Brown said Saturday.
ESPN's Jeff Goodman and Jeff Borzello contributed to this report.