Was hanging out with my brother [he's a pilot for United] on Saturday and I asked him about the Southwest thing. He had actually just been talking to a pilot for Southwest and they talked about it.
Apparently it was not some big "sick-out" protest. There were some pilots who apparently knew that when this mandate came through would be out of a job, and a lot of those pilots that had sick time available burned through it, which is probably where the rumor that it was a big "sick-out" protest came from.
From what he's said, it's mostly structural issues that all kinda bubbled up at the same time. Southwest was already understaffed and having staffing problems trying to come back from the downturn during COVID.
He did say it wasn't weather. That was [as mentioned by others here] just a cover story. But it was covering for the fact that they're having staffing problems, not that they had a vaccine protest in their work force.