Why did the price on these homes drop so substantially?

I was looking at some small multi-families on Zillow, just to analyze prices, when I came across a couple of homes in my area (Durham, NC) that had sold for around 500k in 2018. Now, in 2019, the homes are listed for around 250k, a 50% decrease from just one year ago. This doesn't seem to be a market trend, just a thing on these two duplexes. What could cause a price drop like this?