Here is some of what Maciag says about the overall migration shifts:
For each of the past three years, more people have left the D.C. metro area for other parts of the country than moved in. In 2016, the reported net domestic migration loss topped 31,000 — the steepest decline in years. That represents a stark reversal from the immediate post-recession period when the region enjoyed especially strong population gains. Much of the shift is explained by the economy: The Greater Washington region weathered the recession better than other parts of the country, but jobs have since returned in places that previously sustained severe job losses.
More worrisome for Fairfax County is the fact that its migration loss accounted for more than half of the total negative net migration and, at -17,800 people, was more than double the second worst loser, Prince Georges County.