As everyone else said... it's the carbonation. As the temperature of the soda drops, the saturation level of CO2 drops as well, and the liquid gives up all of its gas. Eventually, the pressure builds *REALLY* high... which normally woukldn't be a problem, except when the liquid then freezes, weakening the bottle and really letting it blow.
What's lots of fun is to take a plastic bottle with a few ounces of soda still in it, and submerge the bottom half of the bottle in a small vat of liquid nitrogen. you get to see the dissolved gas saturation leves change dramatically and I do mean dramaically :-)
(no subject)
What's lots of fun is to take a plastic bottle with a few ounces of soda still in it, and submerge the bottom half of the bottle in a small vat of liquid nitrogen. you get to see the dissolved gas saturation leves change dramatically and I do mean dramaically :-)