Donald Trump showed improvement over Mitt Romney in the rural northern counties, even those that vote Democratic. I think CA-1, 4, and 8 are the only three congressional districts where Trump did better than Romney.
Barack Obama won the national vote 51.1%-47.3%. Hillary Clinton won it by 48.2%-46.2%, a difference of 1.8% in their margins. If you exclude California, Barack Obama won the popular vote 50.1%-48.4%. If you exclude California, Donald Trump won the 2016 popular vote 47.9%-46.6%. That's a difference 3.0% in the margins. Hillary Clinton did do better than Barack Obama in a few other state but in most she did worse. So why did California buck the rest of the country?
It actually didn't. As in California Hillary Clinton showed good improvement over Barack Obama in suburban areas, while she did much worse in rural areas, and the results were mixed in urban areas. She did better in some but worse in others. What makes California different is that it has so many large cities and larger cities have larger suburban areas. California has 4 of the top 17 metro areas. They account for 24 million people and 61% of Californians in 2010. Michigan has only one metro areas in the top 65 and it accounts for 44% of the state's population. North Carolina's largest metro area is 34th. Arizona has 67% of its population in the Phoenix metro area and it was one of the handful of states where Hillary Clinton improved over Barack Obama.
I don't have the data for this but I think if you look at cities and counties by population density you'll see the biggest improvement for Clinton over Obama in the high population density areas and the biggest for Trump over Romney in the least dense areas. The results will likely show that the relationship between the 2016 and 2012 votes is fairly similar across the country if you look at density. It's just that California has more population dense area than other states.