He's probably aroused by his own body, like most of the athletes, by the idea that he's almost naked in front of a large audience. An ego booster for him.
I think it has also to do with seeing the reactions he gets from the audience as he poses and flexes regardless of their sex. He probably think he is hot himself but seing it confirmed on stage may eroticise him.
Well, not the first time I know some many bodybuilders find the act of getting on stage sexually arousing.