Southwest suffered similar widespread disruptions over several days in June, attributed to technological problems, both internally and with a third-party weather data supplier. Hence, the delays preventing crews from reaching flights they were scheduled to work.

Southwest’s pilot’s union said it was aware of the widespread cancellations but did not attribute them to pilots staying home. However, the union has objected to the airline’s recent announcement that it would mandate vaccination against the coronavirus.

“We can say with confidence that our pilots are not participating in any official or unofficial job actions,” the union, the Southwest Airlines Pilots Association, said in the statement.

The union said that its members “will continue to overcome” management’s “poor planning” and other outside challenges, adding that Southwest pilots “always maintain the highest level of responsibility to their crews, their passengers, and our airline.”

The pilots union did say while it does not oppose vaccinations, forced vaccine mandates could lead to labor shortages and flight disruptions. The union is suing the airline for taking a series of “unilateral actions,” including the vaccination requirement.

United Airlines, the first large U.S. carrier to impose a mandate, said recently that:

“nearly all of its 67,000 employees had been vaccinated, except for about 2,000 who had applied for religious or medical exemptions and fewer than 250 who face being fired for failing to comply.

American Airlines, Alaska Airlines, and JetBlue Airlines have since followed suit.

Delta Air Lines has said it will not require vaccinations but will charge unvaccinated employees $200 more a month for health insurance.