Cross bridge formation between myosin heads and actin molecules is caused by the elevation of calcium ion concentration in the cytosol. during rigor mortis, this elevation of calcium ion concentration in the cytosol is permanent because

sarcoplasmic reticulum deteriorates and ATP production is stopped

Explanation:

Rigor mortis is the third stage of death characterized by stiffening of joints and muscles in body. The stiffening occurs because muscles are not able to return to the relaxed state. There are two reasons for rigor mortis, depletion of ATP and increase in calcium concentration in cytosol. Due to these factors the actin-myosin crossbridge is not able to break and the muscles remain in contracted state.

Sarcoplasmic reticulum deteriorates and calcium is released into the cytosol. Sarcolemma ( covering of muscle fiber ) also breaks down releasing extra calcium into the cytosol. Calcium is responsible for formation of actin-myosin cross bridge and when its concentration increases the bridge is formed continuously leading to stiffening of muscles and joints.