Question about pressure buildup in separatory funnel during synthesis of benzyl acetate?

The popping of the stopper is caused by a build-up of gas inside the funnel. When gas builds up, you get an increase in pressure, and "pop" goes the top. The NaHCO3 will get converted to CO2 and that's the gas. It could also be because the organic layer is volatile, and that will cause the pressure to rise also.