Bleeding. Our bodies have a system of blood and tissue factors and cells (platelets) which cause our blood to clot with an injury, so that bleeding stops. Abnormality of any component causes a clotting disorder. Most common is excessive bleeding, as in hemophilia or vonwillebrands disease; you can also have excessive clotting, or what is termed a thrombotic disorder.
Mixed causes. A clotting disorder is due to a defect in the complicated biochemical process of blood clotting. Some of these diseases are genetic, hemophilia (factor viii deficiency) is famous because it ran in the royal families of europe. You can develop a disorder of clotting from medications too, like aspirin, which is a problem before surgery. See http://1.Usa.Gov/afuwso.