Quoting a piece on PlaneBuzz that quotes Lehman Brothers analyst Gary Chase and confirmed by checking Southwest.com:
Southwest has suspends seat sales (not the flights) on the following flights.
Beginning with flights on Oct 4th the following flights have had seat sales suspended:
Oakland to Philadelphia’s only non-stop
Philadelphia to Oakland’s only non-stop
Oakland to Baltimore’s only non-stop
Baltimore to Oakland’s only non-stop
Both of Southwest’s Los Angeles to Philadelphia non-stops
Both of Southwest’s Philadelphia to Los Angeles non-stops
Both of Southwest’s Los Angeles to Baltimore non-stops
Both of Southwest’s Baltimore to Los Angeles non-stops.
It is possible that flights that link these legs, have also been suspended as well.