Southwest Airlines cancelled hundreds of additional flights Monday after a weekend of severe service disruptions.

Southwest had cancelled about 365 flights by Monday morning — 10% of the day’s schedule — and over 600 were delayed.

Dallas-based airline had 1,900 flights canceled on Saturday and Sunday due to weather and air traffic control problems.

Southwest Airlines’ explanation was rebuffed by the Federal Aviation Administration. This agency provides air traffic control services. Southwest Airlines was the only airline that reported such a high percentage of cancelled and delayed flights during the weekend.

Southwest Airlines Co. shares briefly plunged more than 4% Monday, but they recovered somewhat and were back at 2% late in the morning.

After the union representing its pilots requested a federal court to stop the airline from ordering that all employees be vaccinated against COVID-19, the widespread disruptions started. Although the union claimed it didn’t oppose vaccinations, it stated in a Friday filing that Southwest must negotiate before it takes such a step.

The full extent of Southwest’s operations meltdown was revealed over the weekend. Pilots’ union denied reports that they were holding a sickout or slowdown in protest at the vaccine mandate. Southwest Airlines Pilots Association stated that it has not authorized and will not condone any job action.

Another explanation was offered by the union: Southwest’s operation had “become brittle and subjected to massive failures under slightest pressure” due to a lack support from the company. The union lamented the “already strained” relationship between the union and the company.

Because airlines get paid through federal contracts, the White House has forced them to adopt vaccine mandates. This makes them subject to President Joe Biden’s order that federal contractors must require vaccinations among their employees.

United Airlines was the first major U.S. airline to announce a vaccination requirement. Even after Biden’s order to federal contractors and large employers, Southwest had not spoken out. Southwest finally told workers last week that they had to be fully vaccinated before Dec. 8 if they want to keep their jobs. Workers may request to skip shots if they are needed for religious or medical reasons.

Raymond James’ airline analyst Savanthi Syth said that the weekend problems at Southwest will cause an increase in Southwest’s costs and worsen union relations.

Southwest has had a difficult summer due to high numbers of cancelled and delayed flights. It announced in August that it would reduce its September schedule by 27 flights per day (or less than 1%) and 162 flights per day (4.5%) from October through November.