Perhaps Buggy got his hands on the last Road Poneglyph? And says to Luffy, if he wants it, they need to win in a Davy back fight battle.
For the last round, instead of the two captains fighting 1v1, Buggy will of course be too scared and instead suggest Mihawk fights instead, at that point Luffy will tell Zoro to take this, and so we get Zoro v Mihawk 1v1. Zoro beats Mihawk, becomes the WSS, and the SHs have the last road poneglyph and head to Laugh Tale!