Their confluence this year, could be considered bad luck or simply a reaping of what he sows--likely a combination of the two
*Overall managing injuries. This is something that Whitt has struggled with as a coach. From the odd public questioning of Cam Rising's doctor last year and not being prepared if he wasn't able to play to overvaluing the tactical advantage sharing updates on injuries. It's clearly been detrimental to the team's cohesion and Isaac Wilson's development. His dogma of being coy with updates cost him some last year, but this year it has been devastating for them.
*Working effectively with an offensive coordinator. He seemed to have a working truce with Andy Ludwig (the second go around) that produced some moderately successful offenses, but the defense has continued to be the calling card of Utah--the offense has always been the ceiling. The fact that the defense has been so superlative and the offensive beyond anemic is an indictment of Whittingham as a head coach.
*Developing meaningful relationships with players. When things start to fall apart, it's really important to be able to draw on relationships to see you through. Whitt has always been distant from his players--which doesn't seem to be as much of a problem when you are generally winning, but has proven to be catastrophic when the losses start to pile up. Contrast with Kalani who went through the most awful stretch of football since 2017, was able to help his team recover and come within a cat's whisker of upsetting Oklahoma and Oklahoma State in their last two games. Almost no player of significance transferred, setting up a historic season for BYU.
*Overall arrogance. He has indirectly communicated his distain for the Big12 very publicly (by speculating about being invited into the Big10 within the next few years) among other things. While there is no way to directly tie those statements to his team attitudes, it's easy to see how is attitude may have influenced the team. And if it is the case, you could imagine the deleterious effect it could have once the losses start to pile up.
*Perhaps the other (and this is the most speculative on my part) is that he simply lacks the self-awareness to know when it is time to hang it up. He has expressed distain for the NIL and how it works, maybe he has just held on too long and that is holding Utah back.