A-List Preferred and A-List members get the next assigned positions.
This is followed by those who bought EarlyBird Check-In. Because Southwest places no limit on the number of EBCIs they will sell per flight, people often get B (and sometimes even C) positions, even after paying the extra fee.
Then, after all of these people, are the positions available for those checking in at T-24.
Another factor would be if your flight is the continuing leg of a connecting flight. Connecting passengers can check in for both legs (including the flight you're on) when they check in at T-24 for their first flight -- meaning they are checking in before you.
If you need a specific seat or assistance boarding due to a disability, consider asking for a preboarding pass at the gate.
Alternately, you can pay $30 or $40 at the gate to upgrade to an open A1-15 position, if available.