What would cause a glass dish to explode in my hand?

I was preparing a Calzone had the whole thing ready to go in the over and wanted to put egg yolk on the top to glaze it. It was a custard size dish at room temp and I put an egg yolk mixed with fork and was spreading it with my fingers(could not find my brush) when the glass exploded all over in my hand. glass splattered all over the place and know there must have been an angel watching over me because I was not cut up at all. I was shook up and still shaken by it happening when I think of what could have happened. The glass was old but there were no cracks or visable damage to it.
Any ideas?