Earlier quoted context omitted.
It does seem like quite an outlier. I found another article that mentions it and splits it by county. http://socialcapitalreview.org/washington-tops-u-s-in-12-yea... It looks like a problem in the western part of the state, but I don't see any suggestion of why that is. *Correction: Meant county instead of country.
This definitely deserves some kind of investigation. There's probably something to learn from such high levels of difference in very close counties : http://socialcapitalreview.org/wp-content/uploads/2013/03/Sc...
I've heard we also have one of the highest rates of multiple sclerosis in the US, but I'm having a hard time finding good data on that.