It's more than that, of course. Money is involved, and a lot. The movie is based on a book, the book already has a sequel. The movie was well received and performed well, so there's going to be a sequel.
Lively and Reynolds tried (and still trying it seems tbh) to wrestle this franchise away from him by painting her as the creative, and he as a weirdo. Even getting down to Reynolds using his leverage coming off of billion dollar wolverine 3, to bark on Baldoni to his face and threaten his career, eventually trying to muscle their mutual talent agency to drop him.
Why she did it exactly, we don't know. What we do know, clearly, is that all the non-professional energy was brought forth by her, and in the full text messages he's almost comically dedicated to his craft and family.