The exact cause of varicoceles is unknown. Some researchers believe that the one-way valves in the veins leading from the testes back to the heart may be not working properly. These valves prevent blood from flowing backwards in between heartbeats. When the valves are not working properly, the blood can pool in the veins and cause them to widen or dilate. The same is true for varicose veins in the legs or hemorrhoids in the rectum.