Throughout my years of running, there were many times where this would happen. I don't remember it happening during a race but during training. I would be running along and either a cool breeze would blow just right, my dick would be laying in a way that it would flop a lot, or getting caught in a rain storm. The rain feels cold but my shorts started clinging to me and showing more. You have time to think when you are running and there are many things that pass through your mind. When your dick catches your attention, it is easy to focus on it and the next thing you know, you are putting on a nice show. I usually wear a jock when I run now. It keeps things under control.