WN had already announced plans to hire 2000 pilots in 2023, so 8000 in total sounds like normal attrition to me. Last month, they were saying they were having trouble filling the 1200 pilots needed for this year.
Here E, here's a little more detailed on the hiring. All airlines are having the same hiring problems with Pilots and even Mechanics currently. There are shortages on Pilot and Mechanic hiring in the industry, we all know this, or should by now. As I read it, SWA is getting ready for the increased traveling coming soon.