Sioux County is extremely Dutch
Okay but are they Amish or something? You're probably right because there was a county in Nebraska (Saline I think) that always voted Democrat, even in Republican landslides, and when I looked it up I found that it had a large Hungarian population. I guess different European ethnicities vote different ways in the midwest.
I live kind of close to there. The answer is they are not Amish, but they mostly belong to ultra-conservative Calvinist sects. That place is kind of like another world. Pretty much all businesses closed on Sundays. Not a single bar or liquor store in the entire county. Much similar to the ultra-conservative parts of the Netherlands (yes, they exist) that vote for theocratic parties.
As for the original post, it's basically comes down to the eastern part of the state still being populist and voting on economics like Northeastern Minnesota and parts of Wisconsin, and that unlike the states to its west, the larger cities are still fairly liberal. They're certainly not Minneapolis or Madison (except Iowa City of course), but definately far to the left of Omaha, Wichita or Sioux Falls.