The effects of gravity and wear and tear cause the disc contour to change shape.

Most disc bulges are asymptomatic.  However, sometimes the bulge can stretch the nerve fibers of the surrounding bone or ligament and cause back pain.

Other times it can irritate a nerve root and even pinch it causing sciatica.  Some disc bulges progress toward herniation which means that the inner portion of the disc is no longer constrained by the outer cartilage ring.  Even at this stage, most patients improve without surgery.

A great resource for more information is http://www.spineuniverse.com, a peer-reviewed website with thousands of articles by physicians for patients.