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Southwest Airlines is pushing back against talk that this weekend’s schedule troubles were tied to COVID-19 vaccinations.

The airline cancelled more than 1,000 flights over the weekend, leading to frustrated passengers and nervous investors. (The stock is down about 2% Monday.) However, despite whispers it was due to a labor demonstration by the pilot’s union, which is opposing the company’s COVID vaccine mandate, Southwest says the problems were due to “disruptive weather” and air traffic control “issues”.

No other carriers saw delays or cancellations anywhere close to Southwest’s numbers, however.

Southwest, on Sunday, cancelled 1,124 flights, according to flight-tracker service FlightAware. On Monday, that number was down to 363 – an improvement, but still considerably more than any other major U.S. carrier.

Southwest’s pilots have asked the courts to block the carrier’s vaccination mandate, saying it has violated terms of the Railway Labor Act, which oversees airline-union relations. That, combined with the many cancellations, led to speculation that there were excessive sick calls this weekend. The pilot’s union, though, said that was not the case.

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

Southwest had previously warned of forthcoming flight schedule cuts, saying in August that it continued to struggle with staffing issues and was attempting to better align its employee operations.

Between Oct. 7 and Nov. 5, however, those daily cuts were only expected to average 162 flights per day.