thebaddestdude saidTo me subconsiously I am thinking about the possibility of seeing another naked man or being seen naked by another man and it gives me an instant chubby. Then having to basically stroke it to wash it adds to the erection. Lol.
I faced this same issue from my teen years onward, in high school and later the Army. For me the issue was more psychological than physiological, as you describe. To counter it I would fill my mind with distracting, relaxing, non-sexual thoughts. Oddly enough, among the most effective was closing my eyes and imagining myself playing my favorite Mozart piano sonatas.
But I think there's also a certain horniness we get after heavy exercise. I'm sure it has to do with hormone release and other substances in the bloodstream that serve to make us more easily aroused. So for some guys arousal in the shower may be multifactorial.