Bruising on the back of the hand in the absence of a cause is considered pathological and may be related to vitamin deficiency and brittle blood vessels. The bleeding is caused by unintentional rupture of capillaries while working with the hands.
A common cause may be the long-term use of certain medications, such as anticoagulant and thrombolytic ones. Leukaemia, malignancy, haemophilia, idiopathic thrombocytopenia and diffuse intravascular coagulation may also lead to localised subcutaneous bleeding.