Because the democratic party is increasingly operating in the interests of rich assholes, and republicans less of so.
The super wealthy areas of Orange County are still fairly Republican. The shift was largely because of relatively lower income non-white residents making up a larger share of the electorate.
Plenty of rich white areas shifted considerably Dem in both 2016 and 2018. Harley Rouda received 46.6% in 82% non-hispanic white Newport Beach. That was absolutely unthinkable to anyone in 2014.