I believe that it has mostly
I believe that it has mostly to do with the rate at which the chemical reactions can occur. Since the electricity is released via movement of electrons through the electrolyte from one electrode to the other (the electrons are produced from the aforementioned chemical reactions), when the temperature drops the chemical reactions don't occur as quickly and reduce current carrying capacity. The name for this phenomenon is the Arrhenius Law. With reduced power output, the vehicle demands cause the battery to work harder and reduce its capacity.