As the weather begins to cool around the country rock chips that have been unattended to become particularly vulnerable. The short and simple of it is that glass tends to expand and contract based on ambient temperature. If there is a rock chip which has not been repaired, your windshield has already been weakened, even if it appears to be stable and even if the rock chip has been there for ages. The physics involved is also simple. The small micro crack caused by the rock chip will do its best to find a path of least resistance to the furthest point on the windshield to relieve pressure. This is why a spreading crack often appears more like artwork as it swirls around the windshield searching for a final resting place -which usually ends up in an auto glass replacement shop!
All of this is actually avoidable most of the time. While the laws of physics will eventually hold up, all you need to do is have the rock chip repaired before physics makes the choice for you.