Looking at the pattern so far, the opponent has played scissors in rounds 2 and 3. There's a decent chance they might continue with scissors for a third time. Even if they're anticipating my next move based on my paper→rock→scissors sequence, they might expect me to play paper next and choose scissors to counter it. Playing rock gives me a good chance to win if they continue with scissors, and at worst would lose to paper if they switch strategies. Since the opponent hasn't played rock yet, this seems like a calculated risk worth taking.