"Who leaves as a SENIOR STARTER??"
This happens a lot. I know it's difficult for a lot of people to think about walking away but it's not too uncommon. Just look at the NFL. Chris Borland retired after one year, leaving millions on the table as a starting LB for the 49ers. Calvin Johnson just retired at 30. And keep in mind these are guys choosing to walk away from millions at the highest level in the NFL - not just a starter on a college team. McMullen doesn't project to have an NFL career, and if focusing on grad school will benefit him long-term, then it's the right decision. Back about 6-7 years ago Myron Rolle from Florida State delayed entering the draft because he was awarded a Rhodes Scholarship. Even though he was drafted afterwards, most feel taking that year off football may have hurt his chances or either being drafted higher or sticking in the NFL. The point is that at every level of football, it's not shocking that players in their prime decide that things other than football are more important and decide to give up football. I don't have any extra insight or know McMullen at all, and have no idea if it has anything to do with coaches or purely an academic decision, but I know this sort of stuff happens in this day and age of football. More and more players are conscious of the effects, their future, etc and if someone doesn't have a future in the NFL, it's not hard to see them giving up their last year of football to focus on something they feel may be their future.