As it turns out, the physics behind the ability of bulletproof glass to stop the momentum of a bullet without shattering is in fact a lot of chemistry. However, this makes a lot of sense considering that a large portion of chemistry is simple physics applied on the microscopic scale. A bullet typical travels 400 meters per second, which creates a large momentum despite the fact that the bullet is only a few grams. Normal glass is relatively fragile, shattering upon impact of most bullets while sending small cracks in a spiderweb shape throughout other types of glass. However, bullet proof glass is both thicker and built out of more interesting materials. A few different types of polymers are used in the construction of bullet proof glass that are designed to transfer to bullet's momentum into vibrations among themselves, which in turn are spread throughout the glass and absorbed. Normally, these vibrations cause the glass to shatter, but the flexibility of the polymers allow it to absorb the shock fairly well while dissipating the bullet's energy in terms of heat energy. The vibrations transfer the kinetic energy to the glass molecules and since temperature is the measurement of kinetic energy, the temperature of the glass increases as well. 

Bulletproof glass is not completely bulletproof though, it is bullet resistant. In addition to numerous armor piercing rounds from high caliber weapons, a lot of "bulletproof" glass products are only intended to stop a certain amount of bullets. One way to counter these measures is to simply make the glass thicker, therefore allowing it to absorb more energy and allow the stuck bullets to affect its performance less. (This is what is done for President). The reason for bulletproof glass breaking down after a lot of bullets is simply the fact that the lodged bullets disrupt the transfer of energy throughout the glass and eventually interfere to the point where the glass begins to crack and then fall apart. If you are attempting to look out for your own safety, you are better off not creating any enemies as bulletproof glass will not solve all your issues, although it could have saved JFK.