I suspect it was one of those "mutual decisions." There were hints in some of the departing statements. Smith said that he and Rivera were going in different directions regarding the defense. But he also said
he did everything he (Smith) could to position Rivera as a HC candidate !!! (What?) Rivera was "gracious" in his departure, would not be baited into negative statements about the Bears or Smith, who let him coach thru the Super Bowl (helps his resume).
Read between the lines and you will see "you aren't incompetent and you haven't broke any rules. You just can't stay here." The Chicago papers say it was
initiated by Smith. Technically, Rivera wasn't "fired" since he was at the end of his contract.
There's a mutual "decision lurking" in everyone's career. The timing is surprising, but Rivera's situation isn't much different than Russ Grimm. I think Riviera is still in play as a HC candidate next season.