The most common cause is stones thrown up from other vehicles hitting the screen and causing small chips which can then rapidly grow into cracks. Flexing of the bodywork can also cause cracks as the windscreen is an integral part of the structure of the vehicle. If you are not aware of impact damage and your car is almost new I would take it back to the dealer quickly. Excessive body flexing is serious and could be caused by defective welds during manufacture - get it checked out. It could also be a faulty screen but this is unusual. Most likely cause is impact damage which is just bad luck.