I think it could be explained. KF was uncomfortable with playing two QBs during the season. He believes that an offense needs a single QB for continuity, stability, and cohesiveness. Jake was this year's team's predetermined QB1--that was the locker room culture throughout spring and fall camp.
KF was committed to having a QB competition for a short period after Pitt, but concerned how that would affect stability and locker room expectations/culture. Jake comes out strong against Indiana and that convinces Kirk to tie his hitch to Ruddock for the season as he had planned. As the season plays out, he sees that Jake is giving him average production, but the offense would still bog down for long stretches and struggle vertically. But KF is not a flexible man, and his decision was made after Indiana, because he thought the team needed clarity on who its QB would be.
The season is now over. The bowl game is essentially an exhibition, and there are 3 weeks a of practice before the game. KF, in his mind, is no longer bound by his decision after Indiana, because stability/cohesiveness factors that KF thought bound him to Jake are no longer in play. KF now gets to make a "second" decision about who his QB should be. And this time he selects CJB to get the first crack at securing the QB1 position.
Is this how every coach makes decisions, no. It it plausible this is how KF might think, I think so.