Map of where "Multiple Jobs Holders" are located in the US. Why do you think the Mid-West has a majority of these workaholics?

This is from the Bureau of Labor Statistics (BLS).  It shows the concentration of people who are considered "Multiple Job Holders". That could be someone with a a full time job and a part time job or 2 full time jobs, or two part time jobs, etc. Some combination there of.

The darker blue the area the higher the percentage of people with multiple jobs.  Notice much of this takes place in the Mid-West. The Farm Belt, for the most part.  The BLS does not breakdown the data to this level (that I could find) but could it be farmers who have to hold down second jobs during the non-harvest time of the growing season?

Just a guess on my part.  What do you think?