There are several theories as to why your joints crack when put under some form of stress. But the leading one is Cavitation. Surrounding each facet joint is a capsule of liquid, called synovial fluid, that lubricates the joints and allows for smooth, comfortable movements. One theory on crepitus suggests that air pressure within the joint is suddenly altered when the joint is cracked, resulting in the formation or collapse of an air cavity in the synovial fluid that produces a popping sound.