I think it was clear Nagatoro started the whole rock, paper, scissors game because she wanted to ride on Senpai's back, she just got more than she bargained for. Cute chapter.
@Cryotic Perhaps Nagatoro is either referring to some trope in romance manga where the guy carries the girl, or she's just fishing for anything that'll make Senpai feel inadequate when it comes to his lack of experience. Even though she's clearly just as inexperienced herself.