CHICAGO (NewsNation Now) — Southwest Airlines has canceled or delayed thousands of flights, leaving customers stranded after a weekend plagued with major service disruptions.

More than 1,900 flights were canceled by the airline over the weekend, according to the online tracker FlightAware. The Dallas-based airline blamed air traffic control issues and bad weather for weekend “operational challenges” that resulted in the cancellations. On Monday, Southwest canceled more than 430 flights and delayed 1,517 flights. On Tuesday, Southwest canceled about 90 flights and more than 1,240 others were delayed.

By Wednesday morning, the number had lowered with Southwest canceling 38 flights and delaying about 130 more.

But what exactly is going on? Why did one of the nation’s biggest airlines face such major delays, stranding millions? Here’s what we know:

What is Southwest saying about the cancellations?

Southwest has blamed air traffic control issues and bad weather for the “operational challenges” that resulted in the weekend cancellations.

Southwest issued a statement to NewsNation on Tuesday, saying that the flight schedule was “more normal.”

As we complete efforts to stabilize our network, Southwest expects a more normal operation on Tuesday with approximately 90 system-wide cancellations out of the airline’s almost 3,300 flights scheduled for the day. The Southwest Team appreciates the patience of Customers, and we extend our gratitude to our Employees who have worked tirelessly to stabilize our operation. We’ve built a reputation around safe, reliable, friendly air travel, delivered with legendary Southwest Hospitality, and we’re sorry to anyone whose experience did not reflect that over the past several days. If Customers require assistance from Southwest, they can utilize one of the airline’s self-service options for convenience or Contact Us via one of the methods listed on Southwest.com.

SOUTHWEST STATEMENT

In previous statements, the company apologized for delays and blamed the cancellations on weather and “other external constraints, which left aircraft and Crews out of pre-planned positions to operate our schedule on Saturday.”

Southwest Airlines CEO Gary Kelly told CNBC the company hoped to return to normal services by Wednesday and apologized for the delays.

“When an airline gets behind, it’s hard to catch up,” Kelly said.

Kelly, who is resigning at the end of the year, said the primary problems began with weather delays in Florida and quickly escalated into massive delays.

“It’s been a really tough time for Southwest,” he said.

What is the Federal Aviation Administration saying?

The FAA, which provides air traffic control services, took the unusual step of pushing back against Southwest’s explanation. 

The FAA said no air traffic staffing shortages had been reported since Friday but that some airlines were experiencing scheduling challenges due to aircraft and crews being out of place.

Have COVID-19 vaccination policies played a role?

Southwest announced last week that its employees must be fully vaccinated by Dec. 8 to keep their jobs. Workers can ask to skip the shots for medical or religious reasons.

Rumors circulated over the weekend that employees had staged a massive “sickout” in protest of Southwest’s new policy. Southwest disputed those rumors Monday morning.

Southwest had remained silent on the vaccination requirements even after President Joe Biden announced his vaccine mandate order for federal contractors and large employers. United Airlines was the first major U.S. carrier to announce a vaccination requirement in August.

What is a sickout?

A sickout is an organized period of sick leave taken by a group as a form of protest.

What are the unions saying about the vaccination rumors?

As the scope of Southwest’s operational meltdown became clear, the pilots union denied reports that pilots were conducting a sickout or slowdown to protest the vaccine mandate. The Southwest Airlines Pilots Association said it was “aware of operational difficulties” but it “has not authorized, and will not condone, any job action.”

The union offered another explanation for the delays: It said Southwest’s operation “has become brittle and subject to massive failures under the slightest pressure” because of a lack of support from the company. The union complained about the “already strained relationship” between it and the company.

The union said last week it would file a temporary restraining order to stop Southwest from instituting the vaccine mandate.

The president of the Southwest pilots union said during a Monday appearance on “On Balance With Leland Vittert” that the numbers back him up: There was no sickout.

“Looking at our pilot pickup rates, meaning working on their off days picking up extra time, it is almost at an all-time high,” Capt. Casey Murray said. He added that sick time hours were on par with a typical weekend.

The Association of Professional Flight Attendants, which represents more than 24,000 of rival American Airlines flight attendants, weighed in on the situation.

“While APFA believes vaccination is our best defense against illness or death from COVID-19, we fully recognize that some of our members have very strong concerns or beliefs regarding the COVID-19 vaccination,” APFA said in a statement to NewsNation. “While being vaccinated is not open to negotiation with management, we will make sure the company abides by the Executive Orders and the applicable reasonable accommodation provisions.”

Have other airlines canceled flights?

Southwest was the only airline to report delay issues on a major scale.

What was the weather like?

A severe weather system hit Oklahoma late Sunday night, bringing heavy rain, lightning and wind to parts of Arkansas, Kansas, Missouri and Texas.

Has Southwest canceled flights like this before?

Southwest had struggled all summer with high numbers of delayed and canceled flights. In August, it announced it was trimming its September schedule by 27 flights a day, or less than 1%, and 162 flights a day, or 4.5% of the schedule, from early October through Nov. 5.

What are your rights as a passenger if a flight is canceled?

The Department of Transportation says on its website, “Airlines are not required to provide passengers with money or other compensation for costs that fall outside of the canceled airline ticket and fees tied directly to the airline ticket.”

This means for passengers on canceled or delayed Southwest flights that hotels, meals and compensation for missing events are not covered.

Airlines are also not required to rebook you on another competitor’s flights.

“Ask the airline or check online to see if another carrier has open seats and then ask if the first airline will transfer your ticket to that carrier,” the DOT advises. “Remember, however, that airlines are not required to do this.”

Passengers on those flights are entitled to a full refund or a rebooking of their flights on another plane from that company.

Refunds also include non-refundable flights if the airlines cancel the scheduled departure, the DOT said.

The lack of passenger rights in this area is due to the policy of Force Majeure, which means airlines are protected from liability in everything from weather issues to labor disputes to mechanical errors impacting flights.

This story is developing. Refresh for updates.