Why Seattle? Because they failed once there, thanks to some alert border guards. They hate failure. They go back again and again until they succeed. They go back like a dog goes back to its vomit.
The Millenium Bomber was heading to LAX....not to bomb the space needle.
Seattle was only the port of entry. The real target for that guy was LAX International airport. My dog never went back to her vomit. EeeeeeYUK.
I understand returning to NY that is/was a high value target for a great many reasons. Seattle isn't unless they hope to start an eruption of Mount Rainier and take out a larger chunk of the area.
More motive yet, they may want retaliation for the coffee shops on every other corner and expresso bars everywhere a shack or cart can be placed in virtually every city in the nation no thanks to Seattle!