Statistics.
There are more 737-800 flying out there than anything else. Windscreens can age and weather conditions such as cold snaps can also influence reliability.
The screens are multi layered with glass and Perspex sandwich construction in aircraft screens. Cracks happen. The more there are up there, the more it will happen to a particular type.
As there's a 737 taking off somewhere every minute on average, you've got 1,500 flights a day to go on.....