I think it's as simple as it looks. AM and his camp see the type of ball dominant guards we are pursuing in the transfer portal (TyTy, NB, Franklin) and along with Curbelo it makes it pretty clear to them that AM will not be Ayo's heir apparent.
I don't know if it is something the coaches see in his game, but he clearly expected to be handed Ayo's role after this year and those targets we are after raise legitimate questions.
I hate to see AM go, but I absolutely trust BU and his staff to build the roster they see fitting their game plans.
That's my hunch as well. He didn't have the kind of 1 and done type of year I'm sure he wanted. Didn't have big flashes of an NBA game. He's only a freshman, but needs to work on driving, finishing, and getting off his shot against good defenses in the BIGTEN. Objectively good productivity, but can't imagine he was satisfied, knowing he was well been behind Ayo, and to a lesser extent, Trent and Curbelo. I expect Curbelo would have fed him more if it weren't for our generational interior big (Kofi), and overall strength inside. And if he compares himself to Curbelo, that's a mistake. If he's going to make it as an under-sized SG in the NBA, he's got to get stronger, more explosive, and confident with his moves to take on defenders around the rim.
He was a superstar in high school, with superstar expectations. Probably makes it tougher to deal with adversity. On top of that, Curbelo outshined him more often than the other way round. For all Curbelo's assists, I don't remember many that were kick-outs to Miller. I'm sure the staff was wanting the ball to go inside whenever possible, and with Ayo as a primary scorer, that would put Miller pretty far down the list.
Whatever his reasoning, hope he's not another Mark Smith --more productive his soph year, but not challenged enough to get where he needs to be. Regardless, I wish him nothing but the best. Really glad he played at Illinois, and he can be proud of being a starter on a BTT champ.