Here is a link to a video of the effect: http://www.youtube.com/watch?v=SfkHS_58yvs
I did some Googling around and searched here and the answer given to this effect is generally that the beer is supercooled and opening relieves the pressure allowing the beer to instantly (or near instantly) freeze.
Ok, I get that except a friend was just describing seeing this and swears he did not open the bottle. Further if you look at the video the beer is still capped and they induce freezing by clinking the bottle on a hard surface.
So what is making this happen here? If anyting one would think clinking the bottle would have the effect of increasing pressure as you agitate the liquid (like shaking up a soda to make it explode on someone when they open it).
I’m at a loss.