Instant Beer Freez

It’s probably just been supercooled - when you super cool something the slightest vibration (when in the lab you take a glass stirrer and scratch it on the glass container) will cause the material to instantly crystalize. In the case of the beer, the reaction simply goes slower than water.

could it have something to do with the bottles being supercold and still having the caps on them?

You can do the same thing with a soft drink like Coke or Pepsi. I usually use a clear or yellow soda like Mountain Dew. For these drinks you gently put an unopened 20oz bottle in the freezer for just over an hour (depending on the freezer). When you take them out after the hour, the soda should still be a liquid. If the soda has frozen, try again with with a new bottle with a little less time in the freezer.

Set it gently on a flat surface and open the bottle. If you are careful enough, the release in pressure should raise the freezing point just enough to make the soda freeze from the top to the bottom in about 20 seconds or so

My guess is that it’s just some powder in the bottlecap that turns the liquid solid and gel-like. You can see the “freezing” starts from the top and goes down through the liquid.
I’ve seen such powder in action as a practical joke on in a friends cup of coffee while he looked away.
but, of course, I could be wrong

Oops, forgot the opposite. Superheated water in a microwaved coffee cup is quite a show too. On removing a superheated cup of water (or coffee) from a microwave there are no bubbles, no sign of boiling. The water is still. As soon as a grain of instant coffee (or sugar) hits the liquid the cups boils over. {Warning: you can get badly scalded playing with this one.}