Why does Soda Pop after Shaking
Soda drinks are carbonated meaning that carbon dioxide is dissolve into it. When carbon dioxide dissolves, the following reaction occurs: $$\ce{CO2(g) + H2O(l)~ \leftrightharpoons ~H2CO3(aq)}$$ As you can see, carbonic acid (a weak acid) is formed. It this which is responsible for the 'fizziness' of the drink. 

Note that this reaction is in equilibrium, meaning that not all of the carbon dioxide dissolve to form carbonic acid, but only a certain percentage actually dissolves. Usually, the percentage of carbon dioxide that is dissolved is constant, however this can be changed by doing certain things to the system.

For example, by decreasing the pressure the equilibrium will shift to the left, meaning that the percentage of carbon dioxide that is dissolved will decrease. This will result in an increase of gaseous carbon dioxide in the bottle. This reason for this is explained by the Le Chatelier's principle. You can also imagine it as a piston pushing the gas into the liquid as shown in the below image. Image (a) is at low pressure while image (b) is at high pressure.



Normally in soda bottles, the pressure inside the bottle is relatively high to ensure that more carbon dioxide dissolves into the drink. So when you open a soda can you decrease the pressure. This results in more carbon dioxide gas to produced. It is the release of this gas that causes that pfffftt sound.

Now when you shake the bottle, due to turbulence, small bubbles containing carbon dioxide form.The formation of these small bubbles allow even more carbon dioxide gas to form. So if you were to shake a soda can and then open it soon after, the soda will jump up quite violently as there is now a lot of carbon dioxide being released. 

However if you were to shake a soda can and then wait a while before opening it, it most likely just create the normal pfffftt sound. This is because you give the system some time to go back to its original equilibrium position, meaning that some of the carbon dioxide gas will redissolve back into the solution .