Racial demographics were a major factor in the partisan shift of these counties, but it cannot be denied that there was a substantial number of voters-moderate suburbanites who voted for Romney and/or George W. Bush, who moved towards the Democrats in recent years. In Orange County, and in San Diego County, this is certainly true. Even in Cobb and Gwinnett, there were many Romney-supporting suburbanites who shifted Democratic in 2016 and particularly last year. Henrico and Prince William are counties full of moderate suburbanites who have soured on Republicans in recent years.
Gwinnet swung to the left between 2008 and 2012, so it's mostly demographic changes
