I have no idea. I doubt it was testing the route unless it knew whether Southwest had amazing success on the newly introduced MSY-SAN route or not (I don't know if they did or not). It could have been a charter, but with summer winding down I don't know what for.
When did this flight leave MSY and arrive in SAN? I checked real-time flight tracking for both cities and the airline and could find no mention of a return flight.
Thanks for alerting me to this.
San Diego, CA