It may have had something to do with the fact that Butler did not travel with the team to Minnesota because he was being treated for flu-like symptoms back home. He joined the team a day later and played on a limited basis in practice---and said he did well in the practice and was starting to feel better. But, Butler's illness gave the coaches a chance to give Eric Rowe a steady diet of first team reps.
There will be conspiracy theories---perhaps Belichick was not satisfied with the reasons why Butler did not make the trip with the team---perhaps they believed that Butler was dealing with other issues.
Funny, but as i was listening to an interview with Butler during Super Bowl week and how he was candidly open about this not being his best season, I thought, knowing BB, that kind of openness with the media does not sit well with the HC and the Patriot Way. Perhaps this was an additional factor.
The fact that the Patriots didn't even give Butler assignments on the sub packages, in favor of playing #3 CB Rowe and #4 CB Johnson Bademosi, indicate that keeping Butler on the sidelines, who had played 98% of the defensive snaps during the year, is far more than a "football decision."
As for Butler's value as a UFA---sportrac rated his market value at $13M a year last week---but, it's very likely that Butler will have to settle for less per annum than that. Much could depend on whether the real reasons for MB's benching emerge.