Did you know that Southwest does not assign seats? If they are allowed to board early, they can grab a row, fine. If they board in the last group, then they'll probably ask others to move. And as stressful as flying is these days, I can imagine friction possibly leading to a why-not outburst.
If I were boarded in the last group and there were no aisle seats and only one window seat available by them, I'd be stuck. That should not happen if they are only booking 2/3s of the seats, life is what happens in spite of plans.