I guess their plans must have incorporated some flexibility to allow for weather, possibly this applied to loads as well but probably just coincidence. It's a horrible way of seeing how bad the US economy has been lately, despite Dubya's claims to the contrary.
Long haul, I have also heard that flights that meet with trouble have high no-show rates. The only example that immediately springs to mind is Lockerbie - four days before Christmas, popular flight cos it leaves at 6pm allowing a complete business / vacation day and still gets into JFK by 9. Every other flight around that period was booked solid but PA103 went out with 100 empty seats. Don't really believe it but it's interesting.