Some possible causes for unexplained bruises are aging, nutritional deficiencies, Von Willebrand disease and cancer, explains TheHealthSite. Bruises occur when damaged blood vessels cause blood to leak into surrounding tissues.
Unexplained bruises can occur in elderly people due to years of sun exposure, which results in blood vessel walls becoming weak. Vitamin C, vitamin K and minerals such as zinc and iron are helpful for blood circulation and blood clotting, and the lack of them weakens the body's ability to heal after injuries. Von Willebrand disease is a hereditary bleeding disorder that affects the blood's clotting ability and results in bruises. People with cancer often undergo chemotherapy treatment, which leads to low blood platelets and can cause bruises on a person's body. Unexplained bruises in addition to nose bleeds and bleeding gums are signs of a bleeding disorder, notes WebMD.