They wanted Utah to beat USC, so USC misses the playoffs and opens up a spot for Alabama, TCU or Ohio State
Visit the Ohio State boards on that day, IIRC there are posts with thousands of comments on the P12 CCG.
They were all in on Utah.
Same reason why Utah fans rooted for OSU vs. Oregon in the game you cited.
CFP is designed to create interest from other schools' fans and from around the country. That's the brilliance of the 4-team playoff.
It's what's great about CFB. So many teams are dependent on the outcomes of other games that you find yourself rooting for completely random schools each week.
Utah was the protagonist vs. USC this year and Oregon last year. In 2019, it was OU and Baylor fans rooting for Oregon to beat Utah.
BYU fans can play it down however they want, Utah has been in 3 games in 3 seasons in late Nov. or December where the outcome had a direct impact on the CFP lineup.