That's a very recent development -- starting around 2008. Even still -- beside Austin -- they are barely liberal. You do realize places like SF, Los Angeles, NYC, Philadelphia, Boston, Chicago, Seattle etc vote democratic at rates close to 75% for like 30 straight years.

Even most southern cities like Atlanta, Jacksonville, Tampa, Orlando, and Miami are well into the 60s.

Even if Dallas, Houston, and San Antonio are like ~52% for the last 8 years, they're a bit of an outlier as far as major cities go.

Houston hasn't had a single Republican mayor since 1982. I'm a Houston native so I know that the city has skewed liberal since long before 2008, and while I can't speak to other cities in as much detail, for Houston specifically this shift is not as recent as you're trying to portray.