Common causes of pinched nerves are disc problems, (bulging, herniation, degeneration, rupture) alignment problems and bone spurs (osteoplytes).
Each nerve root exits the spinal cord from the space between each pair of vertabrae. The size of the space for the nerve root to exit is mostly determined by two things: the disc and the alignment of the vertabra.
When a disc bulges, herniates, degenerates, etc, there is less space for the nerve to be "pinched." Misalignment of the vertabra diminishes the space for the nerve root and increases the risk of a pinched nerve as well. Often, both are present, which increases the risk further.
Chiropractic manipulation aligns the vertabra, thereby increasing the space for the nerve to "breath." Pinched nerves not only cause local pain, but pain that radiates to the shoulders and arms or hips and legs. Numbness, tingling, and weakness are also signs of pinched nerves.