Two things could be at work here, or a combination of them
1) Yeast may have still been active and creating more fermentation of sugars. Most yeast pretty much go dormant at refrigeration temperatures.
2) CO2 will not readily stay disolved in a liquid at room temperature. The colder a liquid is, the more CO2 will remain disolved in the solution. So CO2 would tend to bubble out of solution at room temperature, but is much more prone to remain in the liquid at refrigeration temps. This is why when you pour a warm soda into a glass you get so much foam, but the soda tastes flat. It is because most of the CO2 came out of the room temperature soda which is what caused all the foam.
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