Scientists are aware of this phenomenon, although they don't know what causes it. They call it "exercise induced orgasm" (EIO). Most of the articles I skimmed were about women who achieved orgasm this way, but I found one article that said a number of men also experience the phenomenon, mostly when climbing ropes, doing pull-ups, etc. It seems to happen mostly in exercises that involve the lower abdominal muscles, so some people call them "coregasms." The gay porn star Rick Thomas is capable of having orgasms when doing pull-ups, and does so in a couple of his movies: "Beat Patrol" and "What a Man's Gotta Do."
I used to cum that way all the time climbing trees, climbing the rope in Gym class, fuck any time my legs made that action, I busted nut. Few times I was almost caught, but if anyone saw they didnt' say anything.
How was he caught, JJ30?! That would have been so beautiful to see.