From what I've heard from inside the byu administration building, TH let BM know that he needed to move on two years ago.
He was interviewed and had the osu job lined up and was all but offered the position. His professional references included GA. When osu contacted GA he told them he was interested in the position and was hired almost immediately.
Bronco had to come back to TH and ask for another year, hoping that a great year (bcs or playoff) would redeem his reputation with fans and standing with TH and would get him extended.
Bronco wanted to be at byu. TH wants to win and be elite.