You guys had better stop, or I'm going to have to post the entire story of the time I pooped my pants while riding in New Jersey and then had to bike all the way home to Long Island with the load in my shorts.

Anyway, I, too, have this same problem just before leaving work. I think, as corrado33 said, it's the body anticipating the exercise. However, my guess is that it's psychological, rather than bio or physio-logical.