In addition to the Business Select passengers, you would be bumped by the A-list. This is those who have flown more than 32 flights with SWA since Nov 2007 and purchased a discounted fare (because Busienss Select fares already are at the front). As long as the A list member booked 3 days prior to the flight, they will be in the group beginning at A16. Note that within the A-List they are ordered by the number of flights that they have flown in the previous year, so the guy at A16 flew 58 flights last years and A17 flew 55 flights, it has nothing to do with when they booked or when they checked in.

FYI, I flew 159 times with SWA over the last 12 months, I am confident in this process as I have never been behind A16 and I do not check in until I get to the airport about an hour before my flights.