What happens when I store coffee in the refrigerator?
Your post combined with Graham T's answer leads me to believe that brewed coffee has some organic molecules that, over time, break down to react with something in white table sugar that produces a gas.  The thing is there exists over 1000 different kinds of volatile organic compounds in roasted coffee that it would be hard to pin down precisely.

You mention in one of the comments that you're a beer brewer--I assume you're a drinker too.  Are you familiar with Guinness? They normally can their beers with nitrogen for smaller bubbles.  If you can distinguish between co2 foam and no2 foam, that may help you narrow down what's going on here.

As an aside: This is a really cool question and I hope you find a more acceptable answer.  This one sounds like it's pushing the knowledge of molecular gastronamy.