Here is another colored Map of singles like Wednall’s - DC is the second biggest
Some contributing factors:
There is a heavy percentage of African Americans in the DC area and they tend to marry at rates lower than whites - that cultural aspect is probably a piece skewing the results.
I think the DC area’s relatively high affluence factors in - Women who don’t want to marry do not have to.
Generally (even in NOVA) there is no social/cultural taboo on divorce or not marrying at all or living together. IOW it is more socially liberal on marriage issues
Finally I think partially a reason this is happening is that more women, in general, graduate from college than men. There are many higher end jobs on the eastern seaboard, especially in the northeast megatropolis, that tends to attract folks from other areas, jobs that you need a college degree to get into/succeed in. So if you move to [in this case] DC to use your Law degree, or MA, or BA or PHD and 62% of you are women, and that goes on for a decade or more eventually that makes a notable demographic “tick”.