I'm going to add some speculation to this, since it is my thread.
I say that:
1. When Rex was hired, one of the points that Whaley brought up was that there needed to be an open QB competition. He couldn't get away with insisting that EJ be made the starter going in, but he did insist that he needed a chance to win the job.
2. Rex agreed to this, knowing fully well the day that TT was signed that TT was going to be the starter.
3. The open competition turned out to be a charade, solely there to make DW happy. As long as TT looked halfway-decent, he was going to be the starter, regardless of what the other 2 did. And since EJ looked good in the pre-season games, DW was none-too-pleased with the result.
And I say that the Bills would've been much better off if Rex named TT the starter a whole lot sooner.