Reading though the Forums here it seems that Southwest automatically assign seats to passengers who obtain tickets on Airtran flights from Southwest from front to back

Does anyone know when this happens? Does it happen when passengers check in ot at some time specific time before departure.

I ask because yesterday I checked in for a flight today at T-24 and was surprised to find I had not previously selected a seat but was able to select 14c, I normally select seat C furthest forward.

Later when I checked seat availability I noticed that all seats almost back to the exit rows had been assigned.

I had assumed that this assignment was done at T-24, but apparently it is done at some later time or as the passengers check in.