Nolan, one of the weirdest comebacks ever.
Mills, while well funded, isn’t an ideal candidate, but neither was Cravaack.
The 8th. District has experienced some demographic changes over the years. The Duluth area has declined in population and the outlying rural areas are a larger portion of its population. Also, white Catholics in the Iron Range and elsewhere are trending away from the Democrats. There also may be a reaction to the behavior of Somali Muslim immigrants who have settled in the area.
This is an older FR article which will help explain why this district has become more competitive:
http://www.freerepublic.com/focus/f-news/1716234/posts