Lividity in a dead body is caused by capillaries decaying due to de-oxygenation.

Explanation: Lividity is unnatural color or discoloration of the skin after death. It starts within 20-30 minutes of death, but is not visible by human eye until two hours. Amount of reduced haemoglobin determines the intensity of color. Capillaries bursts because of increased gravity. The body parts which are in contact with any object or ground are not affected by this because capillaries are compressed.