The skull and crossbones gesture originated, or at least were made famous by, the blackshirts defense of those great University of Nebreska teams
since then, its been emulated by defensive players across the country. We did it on my defense when I played D-III football back in the day...
From Wikipedia: "The logo of the Blackshirts, the starting defensive unit for the Nebraska Cornhuskers football team, is a Skull and Crossbones with the skull wearing the team helmet. The players and fans often celebrate by "throwing the bones", where they cross the forearms in front of the chest, in a 'X', imitating the logo."