The one thing that their main 321 destinations have in common are that they are both slot-restricted. AS has 12 slot pairs at JFK and technically 9 slot pairs at DCA, but only 5 of these pairs are beyond-perimeter. So this would appear to be their attempt to increase capacity to these places, especially for DCA since apparently a 739 needs an average runway length longer than what exists there so they have mainly been using 738s in their "Proudly All Boeing" days.