Leading theory is the POP is caused by cavitation that releases gasses to fill the vacuum that occurs when joints are forcefully separated by manipulation. Other theories that are thought to create the POP include separation of articular surfaces, and breaking of adhesions.
Along with the cavitation we also see a post cavitation refractory time. This refectory period ranges from 15-30 minutes. It is believed to be due to presence of collapsed gaseous nucleus or micro bubbles in synovial fluid which remain unabsorbed in the joint. The gas expands to prevent a sudden drop in intra-articular pressure thus no cracking noise is heard until the gas is completely reabsorbed.