The TE coach, Pat Shurmur, is taking over as OC. He is familiar with Bradford, and prefers the west coast style offense. Norv's philosophy has always been 7-step drops with routes that take a long time to develop, and the Vikings just don't have the O-line to run that style. In the first 5 games of the season Minny was using more west coast concepts, perhaps in part to ease Bradford into the system. In the two games since the bye, they've gone back to Norv's style, and it just hasn't worked.
There were rumblings in the offseason that Turner was going to make this his final year, so the presumption was the Shurmur would take over next year. We just have a bit of an accelerated timetable now. My guess, as a Vikings fan, is that Zimmer told Turner that he could either resign or be fired as a courtesy, and Norv chose the former.
This is a good thing for this team, and I personally bet Minny -6 the instant I saw the news.