I don't know, I just visited Seattle for the first time last month. I really liked it on the west side of the cascades. It's very pleasant. But as for the people, I didn't really interact with too many of them. It was the biggest city I've ever been to, so it was a bit different for me to see the fast pace of it all, but over all I didn't notice a lack of manners in the people.
Maybe people are looking for information to label the natives of Seattle and that's the only negative thing they could see that was common?
It could be true. I'm also from the Pacific Northwest, but Canada - Victoria (Vancouver Island). I know that in Vancouver at least, people have somewhat the reputation of being suspicious and not very friendly. I wonder if that could lend itself to passive-aggression. I think I can be a bit passive-aggressive myself!