The plan was pretty clearly CM Punk vs. Rollins, Cody vs. Reigns.
However, Punk got injured and can't compete at Wrestlemania, and they appear to have panicked. Thus, they decided to go with Cody/Rollins, since that is the closest thing to the style of talent that the Punk/Rollins match represented, and then they rather blatantly moved up Rock/Reigns. We all knew Rock/Reigns was going to happen, but it seemed more likely to be a Summerslam thing. Punk getting hurt appears to have changed things.
EDIT: My guess is that Rock/Reigns wasn't meant to be for the title, but a "pride" kind of match. However, once Punk got hurt, they decided... Screw it. Have Rock/Reigns be for the title, where Reigns will inevitably win, and figure out what to do after. Meanwhile, still keep the "workrate" Rollins match by putting him against Cody.
I've heard this but with Rock's schedule you have to book his shit months in advance