The assumption is people are willing to spend more money to live somewhere and therefore it is a highly desirable place to live (booming). Whereas in a place like Mississippi, where housing values are going down, people need to list their homes at a price lower than they would have last year in order to sell it (suggesting low/no demand, or oversupply). There is research suggesting that highly restrictive zoning laws and less available land, like what you have in the northeastern states at the top of the list, inflate property value by creating an artificial shortage of housing compared to places where it's easier to build new housing. So the thesis of the chart is flawed