North Dakota's situation involves two factors: one a long-term demographic trend and the second the recent find of oil in the Bakken formation.
Like many rural areas, the Great Plains have been steadily losing people as many younger and working age people leave for jobs, potential mates, and other amenities in bigger cities. The people who remain behind tend to mature into old age there. Thus, this demographic phenomena is called aging in place. Unlike popular retirement states such as Florida and Arizona, the Great Plains' seniors usually grew up and never left the rural communities that are their homes. In much of rural North Dakota, unemployment is low in part because there are few working age North Dakotans to take jobs being vacated by retiring seniors.
Bakken oil field under western North Dakota. In places like Williston, ND, jobs are plentiful and housing is hard to find. Like earlier boomtowns, the discovery of oil has led to a sudden influx of workers and the infrastructure in these rural areas is trying to catch up to all the new residents.