Cucinelli nearly won just four years ago. Something else happened besides the demographic trend, which is gradual. Maybe it was the felons Mcauliffe legalized. Normally I don’t put much stock in simple explanations like that, but this result doesn’t totally make sense to me.
In 2013 McAuliffe received 1,069,789 votes to Cuccinelli's 1,013,354.
In 2017 Northam received 1,389,691 votes to Gillespie's 1,159,758.
So Northam outperformed McAuliffe by 320,000 votes. And Gillespie outperformed Cuccinelli by about 150,000 votes. And Gillespie exceeded McAuliffe's 2013 vote total as well.
In 2012 Romney received 1,822,522 votes compared to Trump's 1,769,443. So Trump actually received 53,000 less votes than Romney. On the other hand, Hillary received 1,981,473 votes compared to Obama's 1,971,820 in 2012.
The bottom line is that the Dem numbers are increasing faster than the Reps in terms of votes. The fact that Northam increased his vote total over 30% compared to McAuliffe is frightening. VA is now a solid blue state.