When I saw that Jokerman posted, I was hoping that ...he would have a scientific explanation for this.
Of course i do. But I decided not to explain because I knew you'd be expecting me to.
Ok, since you need it so bad. Glass is very brittle. It needs to be tempered or annealed, heated and cooled to reduce internal stress. If there's a flaw in the enamel matrix when the glass is made, something which is pretty common and which can't be seen, over time the glass will eventually get more and more brittle from temperature shifts, vibrations, and clinking it against other glass or dishware, until the glass reaches a critical point and fails.
It's just like if you let water drip into a container drop by drop, eventually one drop will be all it takes to overflow the container. For this situation, Rukas touching the glass was that one drop too many. The vibration of the touch (or perhaps heat from his hand) was enough, more than enough, to make the glass explode at its weakest point. So he could have touched the bottom and the top could have broken off. That's where the flaw would have been. Often a glass will "explode" without being touched. Because it's at it's breaking point and a temperature change or vibration is one too many.
And a tiny chip in a windshield should be fixed as soon as possible for it can grow with time and just shatter.
