The Salt Lake Tribune has learned that Williams violated unspecified team rules and was told he could either be suspended indefinitely or withdraw from school. In January 2014, BYU discontinued the practice of acknowledging and publicly discussing honor code violations.
Williams recently deleted his Twitter account, and neither he nor his mother, Nicole, returned phone calls and emails Thursday seeking comment.
A check with various Utah County law enforcement agencies turned up no new brushes with the law since Williams was arrested on suspicion of underage drinking in February 2014. A search of recent Utah district and justice court filings also failed to disclose any legal issues.
Last summer, Williams was suspended for BYU's opener against UConn for what he admitted was a violation of the portion of the honor code that forbids BYU students from drinking alcohol. The discipline was not directly related to the February incident, he said last August.
The Cougars will report for preseason camp Friday and are slated to start practices Saturday afternoon.
Williams' absence this season is a major blow to a team that starts its season Sept. 5 at Nebraska and also plays Boise State, UCLA and Michigan in September, in addition to East Carolina and Cincinnati in October and Missouri and Utah State in November.
In three years as a starter, Williams rushed 492 times for 2,526 yards and 23 touchdowns and caught 53 passes for 487 yards and a TD. He is 930 yards from passing Harvey Unga (3,455) as the school's all-time rushing leader.
Williams' reason for withdrawing is apparently not related to the knee injury that cut short his junior year. He sat out all of spring camp due to the injury after appearing in seven games and rushing for 518 yards and four touchdowns in 2014.
The product of Summit High School in Fontana, Calif., said in June that he was ready to play and had no intention to redshirt.
"Do you expect a fish to swim in water? Of course, bro, " Williams said when he was asked if he expects to pick up where he left off in 2014. "I feel really good, and I am getting to the point where I am getting ready to play and everything."
Quarterback Taysom Hill also suffered a season-ending injury in 2014, but he has been cleared to play after sitting out most of spring camp.
Without Williams, Hill will almost certainly be tempted to run with the football more this season, something he was hoping to avoid after failing to finish two of his first three seasons due to injuries suffered while running.
Senior Adam Hine is listed as Williams' backup on BYU's post-spring depth chart, but the bulk of the carries will likely go to junior Algernon Brown. Brown (6-1, 229) rushed for 324 yards on 68 carries last year, including 79 yards on 19 carries in the Miami Beach Bowl against Memphis. Senior Nate Carter and incoming freshman Riley Burt should also help pick up the slack, along with fullback Toloa'i Ho Ching.
One-time running back A.J. Moore, who recently switched to defensive back, might also be asked to return to his former position. Squally Canada, the highly touted transfer from Washington State, must sit out the 2015 season due to NCAA transfer rules.