This demonstrates the principle of freezing point depression. Pure water at 1 atm will freeze at 0C or 32F. When something is dissolved in it the freezing point drops to a lower temperature. This is why when roads are salted there are puddles even though the temperature is below freezing. The more salt the lower the temperature must go before it freezes. When you open the bottle it does the opposite. The freezing point goes from (a guess) -10C to around 0C when the CO2 leaves the solution (the concentration decrease). Since the soda is at -7C it freezes instantly when the CO2 leaves the solution.