Temp differences between the inside and out along with a chip in the glass.
A rock or pebble kicked up by a car or truck tire.
A bullet.
A twisted frame putting preasure on the glass and a jolt from a road bump.
God trying to tell you...change your underware?
*LOL*
Many reasons why it could happen. Just say Thank you Lord that you are ok. Glass can be replaced, you can't.