Spinal fractures are caused by trauma or by conditions that weaken the bones.
A traumatic event can put too much pressure on the spine, and the vertebrae can break because they can't withstand the force. The trauma can cause your body to move in extreme ways, placing an extreme force on your spine. For example, a diving accident can cause the neck to bend back too far, fracturing the vertebrae as they try to withstand the extreme, unusual pressure.
It's possible for a traumatic event to crack the hard outside part of the vertebral body—the cortical bone. If it cracks, it's possible for the vertebra to continue to collapse, even under normal weight. So—the initial traumatic event may not cause the bone to collapse all that much, but over time, the fracture may become worse.
Conditions that Weaken the Bones
If your bones have been weakened by a condition such as osteoporosis, you're more at risk for spinal fractures. With weak bones, even a simple movement like picking up a bag of groceries can cause a fracture.