This is down to the fact that Zoan Devil Fruits have a will of their own, as confirmed in One Piece chapter 1044. Eventually, this Devil Fruit found its way to Shanks, who took it to Luffy ...

This is evident from the very beginning of the story, and of course, this fact was reinstated by the Five Elders in One Piece chapter 1044 as well. The Elders clearly stated that the Zoan Devil ...