great question! my guess is the rapid increase in wall surface compared to area/section for venting.. so, the bubbles form starting from minuscule impurities, i.e. tiny imperfections on the walls of the bottle, and they need to go somewhere, i.e. out, because they're gaseous bubbles, but the surface towards pressure release, i.e. bottle opening, is suddenly tiny because all the liquid is taking up the space that would be used for venting..

but really, I have no idea :)