I have seen this happen for the last year. I saw it for my trips last year in May & December & for our upcoming trip in April. We fly from Buffalo but when it first happened last year, I checked at least a dozen cities & it was the same in all of them. When they came back for me, I checked & they were back in every city they were previously gone from. I think this is SW's new business model personally. Open low, raise by the middle of the day, take away WGA fares & scare people into booking higher priced tickets. Bring them back randomly then go up & down a couple of times.
It used to be easy to book SW but they seem to be playing a lot of games lately. You can still get very good prices, but it means checking the website like a stalker. They still have several nonstops to & from Buffalo at good times & we just earned a companion pass last month. So we will be flying them at least thru 2019. I still keep playing the game tho to make sure I have the best price for the ticket we are paying for.
I did post about it a couple of times on other threads. Will try to find one of them.