While the windows are more susceptible to damage than other cars, I don't recall hearing of spontaneous cracking. If you check along the crack for a point of origin, you might find a small rock strike. The size and, perhaps, structural nature of the windows (along with having no hood to deflect debris) adds to the susceptibility to rock chips. BB guns are also a problem in some places.
The glass troubles with some early customers is why I carry 0$ deductible on my comprehensive. My insurance will cover glass, if I have an incident, with no out of pocket.