What can cause gl to fracture on its own?

On the evening of a mild 60-70 degree day, we returned home to find the gl deco plate on our wooden end table cracking slowly before our eyes. We noticed it because of the slow crackling and popping sounds it made as the fracture made its way across the center of the plate. It finally fell apart in two halves after about 3-4 minutes. The whole event must have taken 5 minutes total. As it was occuring I inspected it for a chip on the end where the crack began and found nothing. There was no cold draft from the front door. All else seemed stable indoors and out. Weather was perfect. What can cause this to occur, especially at this slow motion like speed?