Each Mentos chewy mint has thousands of tiny pits all over the surface. These tiny pits are called nucleation sites - perfect places for carbon dioxide bubbles in the soda to form.
As soon as the Mentos hit the soda, bubbles form all over the surface of the chewy mints and quickly rise to the surface of the liquid. Couple this with the fact that Mentos are heavy and sink to the bottom of the bottle and you've got a double-whammy.
When all this gas is released, it literally pushes all of the liquid up and out of the bottle in an incredible soda blast.