why did it happen?

An exploding chip is a relatively rare event - if a fault develops in a chip that causes too much current to be drawn the chip may get too hot too quickly and any gasses produced have no way of escaping. Eventually the gas pressure inside destroys the chip package.

Here's a video of a guy deliberately exploding a processor chip by running it too quickly and without cooling.

http://video.google.com/videoplay?docid=5393904704265757054

Unlucky for you a chip failed - it just happened to explode.

Peccavi
March 2009