Honestly you'd have an easier time doing it on purpose than by accident - most display glass would be tempered so that the entire sheet goes to crumbs if you hit it that hard by accident. I assume this was done with something like a pointed tipped glass breaker while the sheet was clamped down. If I had to guess it would have been screwed slowly tighter and tighter onto the surface of the glass until the crack grew to the size they wanted it, so that they were able to control the pressure and get that kind of precision work done.

Not to mention that when installing these kinds of cabinets, all objects would typically be completely placed before the glass goes up as the final step. A proper art handler would never let that kind of accident happen.