It's probably because you're using a button and not a link for the buttons.
So if you change the "Button display method" option in the configuration, it should work fine.
Otherwise, you would have to change the line:
echo $this->cart->displayButton(JText::_('ADD_TO_CART'),'add',$this->params,$url,$this->ajax,'',$max,$min);
like that:
echo $this->cart->displayButton(JText::_('ADD_TO_CART'),'add',$this->params,$url,$this->ajax,'',$max,$min,'icon-cart');
in the file "quantity" via the menu Display>views in order to add the class directly to the submit button.