answer:When heating, you’re essentially increasing the pressure exerted by the air on the glass in which it’s trapped. As the air has no means of escape if enough heat is added the increased pressure will eventually exceed the strength of the glass leading to fractures. If you’re really interested you could probably figure out just how much pressure is building via the ideal gas law. It could also be you’re heating the glass itself too quickly, that’s easy enough to test by just putting in a single sheet and seeing if you get the same results without the air pressure.