The discs, the vertebral bodies, and the neural arch normally absorb compression and load on the spine. The neural arch is a bridge of bone that forms a circle around the spinal cord to protect it.
There is a certain amount of natural degeneration that occurs in the spine starting at age 50. Bone density is less as the bones become more osteoporotic.
In elderly spines the neural arch (instead of the vertebral body) takes up to 90 percent of the compressive force. The discs tend to thin out as we age and don't help absorb the shock as much.
The result is that when the spine is bent forward most of the pressure in on the front of the vertebral body. When the spine is extended most of the pressure is on the back half of the vertebra. Fatigue, aging, and gravity have a tendency to bring the spine into a forward flexed posture. The right angle and the right amount of force can result in a vertebral fracture.
All of these things can contribute to a vertebral compression fracture even in otherwise healthy adults.