I've just found some more theories.
One, that in armour, metal plates would overlap so that a lance wielded by a right-handed opponent wouldn't slide under so easily and into the wearer.
Second, that ladies would ride side-saddle facing the right side of the horse, so their clothes fastened left-over-right to cut down the daughts that blew in from the front.
Third, (probably apocryphal) that ladies would copy that pose of Napoleon with his hand stuck inside his jacket to mock him, so he ordered that women's clothes be made fastening the other way. (Maybe the mocking made him feel a right tit)