Well, I don't know if any of this has any relation to truth, but you asked for ideas, so here is mine...

You have to remember that SAS only has one daily flight between IAD and Europe that is shared for all of the christmas calendar special offers from and to IAD. This means that towards the end of the calendar, the booking class used for the offer (W in my tickets) on the IAD-CPH and CPH-IAD legs tends to fill up.

The problem doesn't seem to be as bad from ORD or EWR, since both of them have two daily SAS flights to Europe. I guess that the flights from SEA are just less busy in general.

If my idea happens to be true, I really can't see how the situation could improve for the remaining days of the calendar.

That is, unless SAS allocates more seats to the W booking class, of course...