The reaction of phosphoric acid (V) to proteins in the milk - they are cut and causes a precipitate
It is a reaction of the Phosphoric Acid contained in the coca cola to the milk. Phosphoric Acid molecules attach to the milk giving them more density and separate out while the remaining liquid that makes up the milk and cocoa cola now being lighter floats on top. The solid matter is basically milk that has been curdled by the addition of the more acidic soda.
