It is not just random mixing of substances and voila, you made it erupt. There is actually a deeper explanation to that. In a nutshell, the primary explanation to the chemical reaction you have just witnessed is that mixing vinegar, which is acid, and baking soda, which is base, results in the formation of carbon dioxide gas, forcing the contents of the bottle out of the volcano. When an acid and a base are mixed together, they react to neutralize each other resulting in production of salt and also in the process, produce carbon dioxide. In real volcanoes, carbon dioxide is also present when they erupt!