The vacuuming up of highly talented, i.e. smart, people, from all over the country, into the megalopolises, where they assortatively mate and form their own "new class", alienated from their origins, and leaving the towns and rural areas without their talent. They assortatively mate, but then have few children.
(Thus reducing average IQ levels.)
Other IQ shredders include the universities, though there's a lot of overlap between them and the big cities. It doesn't even seem to matter whether the students are in STEM or ethnic grievance studies, the results for fertility are the same - although the results for IQ are better or worse depending on field of study, since the STEM kids are much smarter than those in the humanities.