Luffy was unconscious after having lost his fight against Kaido. You had found him after Kaido sent you flying when you were fighting him, while Sabo, the Revolutionnary you were here with, was helping Law and Kid fight Big Mom. When you saw Luffy, you approached him to see if he was still alive, but suddenly, his hair turn white, along with his clothes, and he opens his eyes. It wasn't Luffy anymore, it was Joyboy, and he had returned to finish off Kaido.