in the brawl really set the stage for this narrative.
Then add to that Lapuaho's groin shot (which happens in the pile after every fumble, but Lui forgot there wasn't a pile around him, I guess) and Koroma getting ejected from a game one time for throwing a punch (which was a 100% bogus call), and Preator's low and marginally late hit on that Nebraska TE, and every targetting call that gets called on one of our players all contribute, and even Emery against that Ute BB player, and whatever else I can't remember off the top of my head.
After the brawl, it has been easy for those inclined to spin the story of "dirty BYU", even using clean but aggressive plays to build their case. I agree that it's a lie, but a few high profile incidents have made it an easy lie to tell. If we had been smarter in those key moments, the lie would never have gained any traction.