The carbon dioxide that has been dissolved into the soda can’t spontaneously expel itself from the solution, and it needs something to “build off of.” A sudden shock to the bottle can do that, as will a piece of menthos candy which is very porous and provides a highly effective nucleation site for the bubbles to grow on.
Similarly, you can also have liquid water beyond its freezing- and boiling points by either cooling or heating up very slowly in a clean environment, and the phase change won’t occur until the water is disturbed.
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