It's most commonly an overuse injury. The most common site is somewhere in the lower extremities. When you run, you create forces two to three times your body weight on your lower extremities. If you're running a marathon, you've subjected your feet and shins to tons and tons of force. Other sports in which stress fractures can occur in the upper extremities include gymnastics, where we see wrist and forearm stress fractures.
Foot fractures are most common in track and field athletes.
We took a group of distance runners with stress fractures and tried to find a common component. The one factor we came up with was that hyperpronation contributed to an increase in stress fractures to the foot. Other studies have identified a more rigid, high arch as a factor. Realistically I don't think there is any one thing across the board that predisposes. In women, osteoporosis can contribute, in which there is a loss of calcium and a weakening of bone.