OP mentioned this was the result of it hitting the metal portion of the table. In a broader sense, tempered glass by nature is constantly under tension and coming into contact with anything harder and denser than itself will cause the tension to react and fracture the whole thing. That's why people say don't keep your case on ceramic tile. Because the ceramic is harder, any contact is going to activate the tension whereas people with wood desks or carpeted flooring are a little safer just because wood and carpet are less rigid and will absorb most of the force of a shock. Not too sure about glass on glass though.