That's a good question. Both states trended to Hillary and Washington actually swung to Hillary. To be honest, I didn't study this topic too closely, but I do have some questions.
I'm not surprised that Hillary swung King County her way (basically, one-third of the state voted for Obama by about 40 points in 2012 and Hillary pushed that margin to nearly 50 points in 2016), but the Tri-Cities in Eastern Washington was quite surprising. For those from Washington, is that a result of the relatively high Latino population? For a Democrat that got crushed in rural America, Hillary seemed to do pretty well in Eastern Washington overall. In the Pacific NW, it was SW Washington and NW Oregon that swung hard to Trump.
Yes there are tons of Mexicans in eastern Washington and if turnout is high among Mexicans then eastern Washington will go democrat eventually
As for southwest Washington the key to the yuge swing was cowlitz county plus grays harbor and pacific counties which voted republican for the first time since 1928 and 1952
But the reason those places voted republican was union blue collar works
Update
So doing more research on these areas I mentioned from 1892-1928 it seems that grays harbor was dark red or blue on this site
Then the from 1932-1948 solidly democrat but from 1952-1960 it was a swing county then from 1964-2012 it was solidly democrat not even voting for Nixon or Reagan
But then trump came and just like the rust belt in the Great Lakes area these county's i listed are the rest belt of the west coast.