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

Livor mortis refers to the bluish-purple discoloration (lividity) under the skin of the lower body parts due to gravitation of blood after death. ... Dark pinpoint spots (Tardieu spots) are seen when capillaries rupture due to increased gravitational pressure (e.g. in the legs of a hanged person)

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.