We open the cap of a cola drink why does excess of bubbles come out?

A cola (or carbonated) beverage is produced by dissolving carbon dioxide in the drink solution under pressure. Increase in pressure increases the solubility of gases. 

Therefore, more carbon dioxide dissolves at the higher pressure. When this pressure is suddenly released, e.g. by

removing the cap of the bottle, carbon dioxide is less soluble, and it comes out in as bubbles from the solution.