Military exercises and an understaffed air traffic control center coincided Friday with thunderstorms, forcing Southwest, United Airlines and American Airlines to cancel flights. But while other carriers recovered Saturday, cancellations at Southwest surged. 

“(Air traffic control) issues and disruptive weather have resulted in a high volume of cancellations throughout the weekend while we work to recover our operation,” the airline said on Twitter on Saturday. 

That prompted a rare Twitter clapback from the Federal Aviation Administration. Adding to the fracas, Republican politicians and pundits falsely alleged that Southwest’s meltdown was due to pilot’s protesting the company’s COVID-19 vaccine mandate, which both the airline and its pilot union denied.

By Monday, Southwest had cancelled more than 2,500 flights. All other United States airlines combined cancelled about half as many flights during the three days, according to aviation analytics company Cirium. So what really went wrong at Southwest?

Southwest soared from regional upstart to one of the largest U.S. carriers thanks partly to flight operations tightly tuned to maximize efficiency and minimize costs, and an aggressive business strategy that has used industry downturns to grow. The airline even has stayed profitable through the pandemic while competitors have been hammered.

But last week, that approach amplified Southwest’s Florida headaches, causing chaos across its network. About half of its jetliners pass through Florida daily and 25% of crew changes happen there, according to Southwest.

Friday’s problems snowballed into Saturday with many flight crews and airplanes at the wrong airport. The airline scrambled to move pilots and planes, but its lean flight operations left it with too few extra pilots and jetliners to quickly contain the disruptions, industry experts told Fortune.

Unable to rapidly add operational capacity, Southwest had to cancel more flights. The airline appeared to be mostly back to normal by Wednesday. 

Southwest canceled a similar number of flights in June, when it struggled to handle a resurgence in air travel, bad weather, and tech glitches.

Other major U.S. airlines struggled too at the time, and in early August, severe thunderstorms in Dallas forced American Airlines and Spirit Airlines to cancel hundreds of flights. However, in all those cases, the airlines quickly bounced back. 

Southwest’s flight operations and business strategy have helped it post a profit through the COVID-19 pandemic, but they also left the airline more vulnerable to widespread disruptions. 

Most major airlines have hub-and-spoke route networks, meaning they funnel passengers from smaller cities through a handful of hub airports connected with more frequent flights on larger airplanes. It simplifies route networks and allows airlines to centralize operations, among other benefits. But taking connecting flights is less convenient for travelers. 

Southwest avoids hubs and uses a point-to-point model to connect smaller cities with nonstop flights. Since the early 1980s, the airline has used that approach plus cheap tickets to grow from a regional carrier into a major player. 

Point-to-point networks are more complicated and several unexpected events collided last weekend in Florida, said Henry Harteveldt, a travel industry analyst with Atmosphere Research: “Southwest absolutely melted down Saturday and Sunday.”

With hub-and-spoke networks, crews tend to live around hub cities, and it is easier to move them around even when there are canceled flights. Most airlines also have crews switch airplanes between flights. Southwest typically schedules crews with the same airplane throughout the day, which makes for quicker turnaround times at an airport, said Mann. It also means the crew hopscotches around the country with the planes, whereas aircraft in a hub-and-spoke network orbit around hubs. 

It’s efficient when it works, airline expert Robert Mann said, but when there’s a big weather problem, crews can get stranded. “If a flight doesn’t operate, now you don’t have the aircraft or the crew for the rest of the day. You have to find replacements to pick up later legs in the day.” 

Airlines have flight and cabin crews on reserve to jump in if needed. With its cost-saving focus, Southwest has one of the industry’s lowest pilot reserve rates, about 15%. That’s as little as half of other major carriers, Mann said.

Southwest lost thousands of workers through attrition last year, including more than 600 pilots, leaving it with 10,000, he said. However, the airline has never laid off or furloughed workers since its founding in 1971. It dropped 195 routes that relied on business travel, which dried up with the pandemic, and has focused more on leisure travel, which has recovered faster. 

As other airlines shed flights, Southwest also moved into airports it had been crowded out of before, such as Chicago’s O’Hare International Airport. So Southwest’s network has grown from 1,407 routes in October 2019 to 1,658 this month, Bernstein analyst David Vernon said. (Using industry downturns to expand, such as after 9/11, is another hallmark of Southwest’s business strategy, said Mann.) 

This fall, the company has trimmed its flight schedule and is halfway to its goal of 5,000 new hires. It also plans to train 120 new pilots in December, a Southwest spokesman said.

Nonetheless, short staffing and aggressive growth has worn out flight crews, say officials with Southwest’s pilot union. 

“Our pilots are tired and frustrated because our operation is running on empty due to a lack of support from the company,” union spokeswoman Amy Robinson said. 

Last week, the union asked a federal judge to block the airline’s vaccine mandate. That plus an exhausted rank-and-file puts Southwest in trouble heading into the holiday season, Harteveldt said: “Southwest used to be the case study for how to run an airline. Now, Southwest risks becoming the case study of what not to do.”

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