Why would a mouse's head blow up?

My mouse named Kyo died tonight. The thing is, I don't know why. One minute he was fine and playing around with his brother Haru and the next thing I hear is a loud pop and his head was blown apart. There was nothing in their diet that could have caused this... Is there some kind of illness he could have had that would have caused this? Right now I am really weirded out because he was only nine weeks old...Any ideas?