The route involving Seattle is actually Phoenix TO Seattle, which is somewhat surprising, and means 3/10 of the list puts people in/through Seattle.

That's a surprisingly large percentage / number.

Intuitively, It feels like too many people to either be: - On their way somewhere else (home to BC, hopping the Pacific, from AK to lower 48) - Coming for snow (either ending in WA, or legging to BC,ID,WY,MT) - Coming home from holiday - Subjecting themselves to January in Seattle [1]

… but maybe in aggregate the above add up to our 8–10 position.

Now that I've written the above out, and others have pointed out, I'm recognizing a pattern with Atlanta: we don't see bigger cities as starting points, just their aggregation point (Atlanta). Maybe reasonable to assume the same for Seattle.

-- [1] Not a dig, I live here.

People from relatively wealthy areas flying to Phoenix for the warm weather?

In my experience those are first legs on (cheaper) stopover flights.

If you fly between Denver and (anywhere on the coast) there is always some cheap option that stops over in Phoenix.

Perhaps it is the same with Seattle going east ...

Phoenix is an American Airlines hub.

Ski traffic to Denver.

Probably snowbirds escaping down to AZ for golf weather and government that's not trying to destroy small business and restaurants.