The London Economist blogger A.S. interprets it (needless to say) as evidence of tax flight, citing net migration into Travis County, i.e. Austin [corrected from Houston in original, sorry—pb] in low-tax Texas, and out of Santa Clara county, i.e Silicon Valley in high-tax California. But the driver could just as easily be flight from immigration. For example, Los Angeles shows massive out-migration to other parts of California as well as to relatively high-tax (but still American) states of the Pacific North West.
There's also extensive out-migration from Miami-Dade county, in low tax Florida.
This flight from immigrant-impacted areas was first identified, I believe, by demographer Bill Frey working off the 1990 Census. He even called it the "flight from diversity". I discussed his work in Alien Nation in 1995, and even constructed a map (p.70-71) with this helpful rubric:
"ALIEN NATION: American is polarizing—because of immigrant clustering and assortative migration of the native-born."Things have just gotten worse. But worse is better!