Tempered glass, generally tinted, can spontaneously break. Usually the glass is in an exterior position on a building elevation exposed to sunlight. There is a defect sometimes built into the glass and it's called a nickel sulfide inclusion. It starts off as a tiny flaw but when exposed to sunlight the nickel sulfide inclusion grows until the stress created causes the glass to explode. Sometimes, if the glass remains in the frame you'll see a radial fracture pattern. The nickel sulfide inclusion is at the bullseye and can be seen with a microscope.
Also, the glass generally used for the fronts of residential fireplaces isn't glass but a ceramic that can handle high temperatures.