Most common reason for a spreading crack is a change in temperature.
If you have any damage to your windscreen, it is important to get it repaired as soon as possible.
Other reasons the crack may spread include the position of the damage, jarring, and rough roads.
Any pressure on the inside of the windscreen especially near the damage will cause it to spread. Any contamination in the damage, especially liquid, will increase the risk.
Place a sticker or tape over the damage, to keep it dry and clean. It increases the chances for a better repair and extends the life of the screen until it can be repaired.