when you ride or exercise very hard, you ar breaking down muscle tissue, or if you have very thin or dry skin or take a lot of antioxidents your blood vessles (the tiny capillaries) are subject to pressure and possible bleeding sort of a hematoma just under or between the layers of skin. I can get them if I absent mindedly scratch my arms with a fingernail, and my legs are constantly black and blue from this as well as bike geometry and general clumsiness.

just a thought