Disk herniation is most often the result of gradual, aging-related wear & tear called disk degeneration. As we age, disks become less flexible & more prone to tearing/rupturing with even a minor strain or twist.
Most people can't pinpoint the cause of their herniated disk. Sometimes, using your back muscles instead of your leg & thigh muscles to lift heavy objects can lead to a herniated disk, as can twisting & turning while lifting. Rarely, a traumatic event such as a fall or a blow to the back is the cause.