According to Wikipedia it is significantly less white than the neighboring counties in the region. (80% vs 90 and 95%)
But has a significantly larger proportion of people in 20-24 age range due to the presence of The University of Arkansas at Fayetteville. According to the Hill, amongst college students there is a significant preference for Sanders over any other candidate, Biden included.
The latest figures, released Thursday, found that 29 percent of likely Democratic voters attending college or university picked Sanders as their top choice for president. Support for Sanders in this election cycle peaked at 32 percent in April.
Sen. Elizabeth Warren (D-Mass.) and former Vice President Joe Biden were the only other White House contenders to win double-digit support in the Democratic field. Twenty-two percent of Democratic participants picked Warren, while 10 percent chose Biden. South Bend, Ind., Mayor Pete Buttigieg (D) followed with 9 percent and former Rep. Beto O'Rourke (D-Texas) with 7 percent.
It seems likely that this is the source of Sanders' victory in the county.