A disc herniation occurs when the outer walls of the intervertebral disc (annulus fibrosis) tear. Other terms for this are “ruptured disc,” “prolapsed disc,” or “slipped disc.” When outer annular rings tear, the gel like nucleus pulposus leaks out pressing on the structures in the area including spinal nerves. The herniation usually occurs at the posterior lateral wall where the annulus fibrosis is thinnest. disc herniations can be caused by the following:

Degeneration or general wear-and-tear which can break down the walls of the disc.
Repetitive bending and twisting motions especially in combination. These motions place the most stress on the outer fibrous rings.
Discs are most susceptible to injury in the sitting or bending position as the anterior pressure on the disc pushing the contents of the disc posterior laterally toward the weak walls and spinal nerves.
A sudden fall or trauma can result in a disc herniation.
Repetitive stress injuries such as over head activities and heavy lifting.
Genetic factors and poor posture may contribute to disc stress, degeneration and eventual herniation.