The body is designed to withstand a certain amount of physical force - bumps, jerks and falls, but if the mechanical stress goes beyond the body's limit of adaptability, the effects may become stored as body stress.
The cause may be sudden and violent, such as a car accident, a severe fall, of lifting a heavy object incorrectly. Or, there may be a gradual accumulation of milder mechanical stress, e.g. habitually sitting incorrectly, doing inappropriate exercises.
In a severe case the intervertabral disc may bulge or prolapse, and possibly pinch on a spinal nerve.