What causes thrombosis of the superficial dorsal vein of the penis? (the main prominent penis vein)

Blocking blood flow: Use of "cock rings" , cords, or other types of devices that block the flow of blood through the vein for a prolonged time is the most probable cause. There are also blood clotting disorders such as anti cardiolipin antibodies that iincrease the risk of clotting. Inflammation from an infection can also cause clotting/thrombosis.