I think Castellanos simply speeded up the process. O'Brian has always preferred pro-style, drop-back QBs. This, to say the least, is not the way Castellanos plays. He was probably going to transfer out anyway. Now he is on the market for sure.
I also agree that such fits of pique are bad for college sports (and the pros won't put up with them at all) and that Castellanos shouldn't have abruptly quit the team. Something similar happened at Tech with Jeff Sims refusing to substitute during a game. It doesn't help the young man's rep and it doesn't help his attractiveness as a transfer. But … some tema will take him, build their O around him, and succeed; he's that good.