Southwest Airlines canceled hundreds of flights over the weekend, blaming the woes on air traffic control issues and weather.

The airline canceled more than 1,000 flights, or 28% of its schedule, as of 5 p.m. ET Sunday, according to flight tracker FlightAware. That was the highest rate by far of the major U.S. airlines. Next in line were Allegiant and Spirit, which had respectively canceled 5% and 4% of their flights on Sunday, according to the flight tracker. American Airlines canceled 2% of its flights.

Southwest Airlines said in an emailed statement that it had experienced weather challenges in its Florida airports at the beginning of the weekend, which were compounded by unexpected air traffic control issues in the same region. Those issues triggered delays and prompted significant cancellations for the airlines beginning Friday evening.

Southwest Airlines is allowing customers to explore self-service rebooking options on Southwest.com, where they can get updates on the status of their travel.

The most troubling reason is the likelihood that some pilots who oppose Southwest’s decision to mandate COVID-19 vaccinations are participating in an illegal job action where they call in sick or are engaging in a “work slowdown.

Southwest’s woes could linger and affect its fourth quarter financial performance.

All of this is happening as people are in the midst of booking their Thanksgiving and Christmas/New Year holiday travel.

It’s very possible that some people who might normally book on Southwest may see this news and choose to fly other airlines.