Its fairly common, usually happens when its cold. The heat from the defrost causes the windshield to slightly expand. A weak spot in the glass, (such as a stone chip), gives a place for a crack to start. Once they start, they usually continue to grow over time.
04 TJ Rubicon, Solar Yellow, 5 speed, 4" suspension lift, 285's(33's), Warn rock crawler front bumper with grille guard, Ramsey 8000 pound winch, JKS UCA's, CV driveshaft