I always assumed it was a cross between the newish tech industry, usual finance industry salaries, fracking, and the ever expanding defense contractor industry that has been generating thousands of millionaires [1].

All of these require an education and most importantly living in the right places. These industries while booming are better at generating upper middle class with a lot of low wage jobs in between, rather than the healthy middle classes common in older industries like steel, mining, and other union jobs that fueled the 1950-70s middle class boom.

[1] https://www.washingtonpost.com/local/dc-enclaves-reap-reward...