Could be either or, but both will be due to Kirei's meddling since he finds Kariya "interesting" and all that.
I'm really confused regarding Rider.. if it was him, his actions would seem rather inconsistent. It's not like Saber wouldn't have duelled him if he asked nicely.
That was nice little red herring from the last preview, with "Return of the Assassin" referring not to the servant class but to Kiritsugu himself.