One of two things happened. They planned for the fans to get all butt hurt over The Rock taking Cody's spot and this new angle was all part of the plan, OR they didn't plan on the fans throwing a bitch fit, and called an audible.
Seems like the latter. Kind of like what happened with Daniel Bryan at WM...30(?), when they had to scrap plans for a Batista/Orton main event, and throw Bryan in there because the fans were going crazy over him getting looked over.
I think they originally planned for Seth/Cody on night one, and Rock/Reigns on night two. Although, Rock/Reigns didn't really make much sense from the get, so they might've had this planned all along.