They are trying to blame it on bad weather but other airlines did not have large numbers of cancellations.
The disparity between Southwest’s operation and other airlines fueled speculation on social media that employees were calling out sick, with some suggesting it was in protest of a Covid vaccine mandate. Southwest said that was not the cause of the mass flight cancellations.
Southwest Airlines cancels 1,800 flights, blaming weather and staffing
Appatrently the pilots are unhappy with the vax mandate but can't say so officially through their union because such a walkout would be illegal. This could easily spread to other airlines, I wouldn't want to be booked on flights right now.