canceled 2,909 flights Monday due to severe winter weather and staffing shortages. While many hope for an idyllic white Christmas every December, this year over-delivered. A dumped snow and plunged temperatures below freezing across the United States. Other carriers were able to weather the storm, but underlying issues at caused an operational meltdown.

The flights canceled the day after Christmas accounted for 71 percent of Southwest’s schedule, according to Delta Air Lines, United Airlines and American Airlines canceled less than ten percent of their flights on Monday. The stark contrast immediately raises questions about why operations at Southwest completely broke down.

“We were fully staffed and prepared for the approaching holiday weekend when the severe weather swept across the continent, where Southwest is the largest carrier in 23 of the top 25 travel markets in the U.S. These operational conditions forced daily changes to our flight schedule at a volume and magnitude that still has the tools our teams use to recover the airline operating at capacity.”

The statement from Southwest doesn’t give a complete picture of the operational conditions that led to the mass cancellations. attributed Southwest’s lack of a hub airport as a factor behind the meltdown. As pilots and flight attendants called out, Southwest’s scheduling system couldn’t keep up and get crews to the airports where they were needed.

Accounts from stranded travelers and a purported internal memo show how the carrier desperately attempted to fill gaps by any means necessary. A memo from Southwest’s Vice President of Ground Operations declared a state of emergency at Denver International Airport. Allegedly, the condition prohibited personal absences, and sick absences required a doctor’s note. Southwest also instituted mandatory overtime and banned shift changes with 12 hours of shift start. Any violations of the memo’s terms would be considered insubordination and result in termination.

The response to that memo? According to a tweet to one stranded customer at Denver’s airport, which included that memo, 120 Southwest employees walked out.

The U.S. Department of Transportation is investigating the mass cancelations and how Southwest handled stranding passengers across the country. “USDOT is concerned by Southwest’s unacceptable rate of cancellations and delays & reports of lack of prompt customer service. The Department will examine whether cancellations were controllable and if Southwest is complying with its customer service plan.” Southwest will likely be dealing with the repercussions of this meltdown into the new year.