What causes degenerative disc disease?

Genetics. In a research study started in 1991 and published in 2009 (the spine journal 9 pg. 47-59), identical and nonidentical twins were longitudinally studied. Twins had widely differing levels of risk factors (activity, smoking, occupation) thought to cause ddd. This study showed that there is a strong genetic component to ddd and that external risk factors previously belived to be important, were not.
Wear and tear. Since we are bipeds and walk upright, our discs support a great deal of pressure. Over time the support fibers (annulous fibrosis) may fray or split allowing the internal disc contents to bulge or protrude. The space between the vertebrae may narrow, and the disc may dry out or dessicate. This eventuates in pain and stiffness.