Often if you get a chip in the windshield, another object striking the windshield afer may cause the crack to occur. Cracks are bad news becasue they will continue to spread until it reaches the end of the glass.
Also, in winter, cracks can start if you have a rock chip and crank up the defrost without letting the windshield warm up slowly.
If you have a crack across your line of vision (the driver's vision), then you are supposed to have the windshield replaced ASAP as this is a drving hazard.