The most common reason for a spreading crack is a change in temperature.
If you have any damage to your windscreen, it’s important to get it repaired as soon as possible. Cracks also spread in relation to their position and any jarring or bumping of the car along rough roads.
Any pressure on the inside of the windscreen, especially near the damage, will also cause it to spread. Finally, a crack that is contaminated (for instance, one filled with liquid) is also likely to spread.
Place a sticker or tape over the damage to keep it dry and clean until you can get it repaired or replaced.