Manufacturing defect, pull the other one. That windscreen goes through a lot of stress and strain, it matters not how old the car is, it is what you do with it. I’m not saying you have done anything wrong, but you might of hit a pothole or a stone hit it at 70+mph, who knows, or when it was parked something hit it. As the other reply says, for the sake of £50 or £100 just claim on your insurance.