Emergent behavior doesn't need to be organized. There doesn't need to be a union nor a secret clubhouse nor an email chain for people who believe they will soon lose their jobs to make use of their accumulated sick leave, especially if it cannot be converted into vacation days, which are paid out when employment is terminated. I don't know what's in the employment contact at SWA but everywhere I've worked paid out unused vacation days but never paid out for unused sick leave. Similar people in similar situations often make similar decisions without coordination.
If pilots being sick caused cancellations they would be on the hook paying for refunds and hotel rooms. If the reason is weather, they aren't. Their incentives here are somewhat obvious.
