It's possible, I suppose, that Milton finally figured out Harbaugh and Co are worthless at developing QBs and that his physical gifts can only take him so far and if he wants to play at the next level he's going to need to be coached up.
Maybe, just maybe, Milton believes Heupel is "that guy" who can mold his raw gifts into gold at the next level, even if he's on the bench a year or two...... I just really doubt he wants to flash signals and wear a headset at UT after doing it at Michigan.
Milton could easily start at...... just spitballing..... even Duke, under Cut, with little pressure except to learn from a great coach, develop his fundamentals and physique for the next level.
I'm not saying Heupel isn't a good QB coach. I think he is. If I were Milton, I'd want game time snaps..... lots of them..... on my resume along with great coaching after I transferred if I really wanted to see the next level.