Why did the glass shatter?

My friends and I were out and sitting at a round glass table. One of my friends emphatically made a statement while pounding his mug on the table. It was kinda hard but not THAT hard….not hard enought where anyone would think the glass would break…yet the glass completely shattered. We noticed he hit the table exactly in the middle and the shatter pattern was circular all the way out. Just wondering if there is some physics explanation as to why the glass would shatter easier if you hit it exactly in the middle?