This is a function of two things. One: A judge illegally redrawing the district lines to be more favorable to democrats.
Two: Obama removed the citizen question. They just counted people in the last census. Consequently, the dems got several more districts in blue Northern Virginia based on people (legal and illegal non-citizens) and not how many citizens.
That is really what the mass migration was about. More census numbers mean more electoral votes. If Trump rightly only counts citizens for the apportionment of electoral votes between states, the dems will lose big time. As many as 20 electoral votes in blue states could go up in smoke. Expect a huge fight.
Obama didn't remove the citizenship question on the census, it hasn't been there since 1950.
Even when the census asked about citizenship, aliens were still counted for apportionment.