According to Sooners Wire, if a team playing against the Texas Longhorns performs the “horns down” gesture, the Big 12 could rule that gesture a taunt in bad taste and players could face penalization. In college football, if the defense has been found guilty of unsportsmanlike conduct, it could result in a 15-yard penalty and an immediate first down for the offense. If it occurs after a score, then the penalty’s enforced at the next kickoff.
So what does the “horns down” gesture mean? It’s basically a signal of defeat. Imagine a bull with its horns pointed down instead of up. It’s the same imagery as a fighter taking a knee or showing a sign of “meekness” or “giving up,” and when a team playing opposite the Texas Longhorns brandishes the “horns down” symbol, it’s perceived as a slight against the team.
It’s kind of like a less extreme version of taunting the Philadelphia Eagles by having one player contorting their hands into a flying bird and the other player pantomiming a gun shooting said eagle. Which probably hasn’t been done because you’ve seen how pissed off people get when athletes don’t stand for the national anthem. Imagine pretending to kill a national bird.
Source : https://www.distractify.com/p/what-does-horns-down-mean