Johnny Manziel is the quarterback at Texas A&M. Their hated rivals are the Texas Longhorns, who have a distinctive hand gesture. Unfortunately, when you are invited onstage at a rock show, you have a tendency to want to be like HEY YEAH ROCK AND ROLL. But you maybe shouldn't do that when you're Johnny Manziel.
See, the problem is that the "hook 'em horns" is indistinguishable from devil horns and there is absolutely no way to extrapolate intent using context. Also of note: the Aggies' rallying cry is "gig 'em," which is of course TOTALLY DIFFERENT from "hook 'em." You can see why the two schools hate one another so.
One of the schools has a dog!