I recently saw something (On Discovery Channel?) about men and testosterone. They took saliva samples from two rugby teams and saw that testosterone levels rose when their team was winning so it's possible that winning can make you hard.
He's standing next to a woman. Maybe she was fondling him sitting down, just before they stood up to cheer. Who knows? But he's at least a little hard. There's also something called a spontaneous erection, that young men get for no apparent reason. Maybe this was one. Not a biggie (nor was his boner).