This is what happened:
The contents were under pressure from the sealed can. The temperature of the beer was sufficient to freeze it, but the pressure inside the can was too much to allow the liquid to become a solid state. Once the can was opened and the pressure was released, the liquid as still below freezing and was now able to freeze. My guess is that is also wuld have bubbled up and overflowed as it slushified (if you didn't start drinking it immediately).
Beer cans tend to be stronger than soda cans, and will not break when frozen, unlike soda cans. Soda's will burst in the freezer to allow the liquid to expand and freeze.
Just call me Mr. Wizard.
