I'm not surprised that he's transferring... these guys want to play and the 2-QB system doesn't jive with everyone. It seems that after spring practice, Tre realized that he was again going to be the back-up/relief QB and felt it better to use his last 2 years of eligibility somewhere where he can be the man.
It's pretty late in the game to transfer but he will have to sit out this year and I'm sure he can find a new home where he can compete for that starting spot.
Best of luck to him, and now IU fans can move forward knowing that this is Nate Sudfeld's offense now. Like someone said before, he can definitely put up monster numbers now that he's not sharing snaps.