A stress fracture will occur in any bone when the repetitive loads on that bone exceed what the bone is adapted to take.
A navicular stress fracture is no different. The bone will develop a stress fracture when the loads exceeds what the bone is adapted to take. The higher loads on the navicular occur from issues with the volume and intensity of activity levels increasing to rapidly for the bone to adapt to them. The loads needs to be increased gradually and slowly to allow time for the bone to adapt to the load.
Loads on the navicular are also affected by the biomechanics of the foot. The navicular is an increased risk in the flat or overpronated foot.
Bone health issues are also important, with vitamin D being implicated in a number of studies.