Southwest Airlines travelers are facing another day of disruptions. The carrier suspended hundreds of additional flights on Monday, after canceling and delaying thousands over the weekend — reportedly due to air traffic control issues and weather.

The airline has canceled more than 355 flights and delayed another 581 as of Monday morning, according to the online tracker FlightAware . Those canceled flights make up 10% of its schedule, as compared to 30% on Sunday.

Southwest said over the weekend that the high volume of cancellations was causing longer-than-usual customer service wait times, and asked affected travelers to explore self-service rebooking options online.

There are mixed messages about the root cause

The reason for the disruption depends on who you ask.

Southwest blamed air traffic control issues and disruptive weather in a statement issued Sunday.

That same day, however, the Federal Aviation Administration said that no FAA air traffic staffing shortages had been reported since Friday.

"Flight delays & cancellations occurred for a few hours Friday PM due to widespread severe weather, military training, & limited staffing in one area of the Jacksonville en route center," it tweeted . "Some airlines continue to experience scheduling challenges due to aircraft and crews being out of place."

Henry Harteveldt, president and travel industry analyst at The Atmosphere Research Group, suggested two other possibilities to the Associated Press over the weekend. One is that Southwest scheduled more flights than it can handle, and operates a "point-to-point route network" that gives each delay a considerable ripple effect.

He also suggested that some pilots are calling in sick or refusing to work in opposition to the COVID-19 vaccine mandate that Southwest announced earlier this month. Southwest executives and employees have denied that, even though the theory has been embraced by prominent conservatives including Sen. Ted Cruz (R-TX).

The Southwest Airlines Pilots Association said on Sunday that "there are no work slowdowns or sickouts either related to the recent mandatory vaccine mandate or otherwise."

It disputed the carrier's characterization of the reason for the weekend's disruptions.

"SWA has claimed that the immediate causes of this weekend's meltdown were staffing at Jacksonville Center and weather in the southeast U.S., but what was a minor temporary event for other carriers devastated Southwest Airlines because our operation has become brittle and subject to massive failures under the slightest pressure," the union said. "Our operation and our frontline employees have endured continuous and unending disruptions since the first time our airline made headlines in early June due to widespread IT failures."

The union said its pilots are frustrated because "our operation is running on empty due to a lack of support from the Company," and said SWAPA leadership will meet with Southwest's vice president of flight operations on Monday afternoon to discuss their concerns.

Southwest has not responded to NPR's requests for comment.

It's been a challenging travel season for Southwest

Many airlines have struggled throughout the pandemic , and Southwest is no exception.

In June, it was hit by a pair of system outages that caused widespread delays over the course of two days. And after seeing an initial spike in demand for leisure travel earlier this summer, the airline cautioned that the delta variant is complicating its plans for a comeback : It warned in August that bookings were falling and cancellations were on the rise.

Southwest announced in early October that it will require all employees to be fully vaccinated against COVID-19 — or approved for a religious or medical accommodation — by Dec. 8 or else face losing their jobs.

"Southwest Airlines must join our industry peers in complying with the federal government's COVID-19 vaccination directive," said Gary Kelly, Southwest Airlines Chairman and CEO. "I encourage all Southwest Employees to meet the federal directive, as quickly as possible, since we value every individual and want to ensure job security for all."

Some employees have already pushed back against the mandate. On Friday, SWAPA asked a federal court to block the company from enforcing the directive, saying it violates the Railway Labor Act (which governs labor relations in the railroad and airline industries).

Southwest disagrees with the claim that any COVID-related changes require negotiation, it told Bloomberg.

The airline has also been dealing with the labor shortages that are plaguing many industries.

Southwest pilots and flight attendants have struggled to find hotel rooms, transportation and meals while on the road.

The Dallas Morning News reports that starting Tuesday, the carrier will give every active crew member a six-month subscription to the delivery service Doordash in the hopes that they can get restaurant meals delivered straight to their hotels.

Copyright 2021 NPR. To see more, visit https://www.npr.org.