Current thinking is that the back is protected by muscles whenever it is pushed off balance or overloaded. The concept of spinal stability, a "steady spine" is in the forefront of research today.
Injury can occur any time the muscles don't protect the motion segments of the spine. Exercise is important for three reasons. It improves strength, endurance, and coordination of each muscle.
Recent attention has been placed on training the "core" muscles of the spine and pelvis. These include the abdominals, hip flexors, and back extensors. The jury is still out as to whether or not a core-training program really makes a difference. Studies are underway even now.
Even if the core training program doesn't accomplish all three reasons for exercising, you'll likely benefit in one or more ways. A physical therapist or qualified fitness professional can help you find areas of muscle weakness or imbalance and design a program to meet your specific needs.