Why does a dead body become stiff?

After death, respiration in organisms ceases to occur, depleting the corpse of oxygen used in the making of Adenosine triphosphate (ATP). ATP is no longer provided to operate the SERCA pumps in the membrane of the sarcoplasmic reticulum, which pump calcium ions into the terminal cisternae. This causes calcium ions to diffuse from the area of higher concentration (in the terminal cisternae and extracellular fluid) to an area of lower concentration (in the sarcomere), binding with troponin and allowing for crossbridging to occur between myosin and actin proteins.

Unlike in a normal muscle contraction, after death the body is unable to complete the cycle and release the coupling between the myosin and actin, creating a perpetual state of muscular contraction, until the breakdown of muscle tissue by digestive enzymes during decomposition. As part of the process of decomposition, the myosin heads are eaten off by bacteria, allowing the muscle contraction to release and the body to relax.