What causes spontaneous glass breakage?

There are several reasons glass might break for no apparent reason. There might be some internal defects that occurred during manufacture or tempering. Several years can pass before the defects cause the glass to break. Chips in the glass edge can also cause a delayed break. Improper installation might be the culprit. Glass edges should not be installed in direct contact with metal as thermal expansion or wind deflection may cause contact and breakage. Those same stresses may cause breakage if proper space for expansion wasn't incorporated. Building settling or shifting as from seismic activity may also put stress on the glass leading to breakage. Glass must also be thick enough to withstand the wind load where it is installed. In any particular instance, it is difficult to definitively determine the exact cause.