I did some research.
What makes traditional Pyrex able to withstand a certain amount of heat without cracking is its low rate of thermal expansion. Borosilicate glass (the original) has a lower rate of thermal expansion than the new glass, and this is the main reason it doesn't crack or shatter as easily with heat.
If one part is hotter than another, for example, it will expand more and there will be stress built up. One part is "trying" to get bigger but it's restrained because it's connected to the other part. If there's enough stress, it will break. In glass that expands and contracts more with temperature, there is more stress.
A scratch might weaken it, but it isn't the main thing.
As far as how it breaks when it does (a simple crack vs shattering), I don't know but of course it has to have something to do with the crystal structure.