What caused thousands of flight cancellations over the Memorial Day weekend?

More than 2,500 flights across the U.S. were canceled over the Memorial Day weekend, frustrating passengers during one of the busiest travel times of the year. 

A range of factors combined to scuttle flights, said CBS News Travel Editor Peter Greenberg, including airlines over-scheduling flights; staffing shortages at airports, airlines and the TSA; bad weather; and a spike in holiday travelers. And as cancellations piled up, that affected people trying to catch connecting flights.

"The real problem wasn't just the flights that were canceled," Greenberg said. "The real problem was connecting flights. Because if the first flight was delayed, that's where people had real problems. They missed their connecting flights. There were no other options. They were stranded."

More than 2.3 million people were booked for air travel during the long weekend, a surge that quickly overwhelmed some airports and airlines, many of which have few staff than before the COVID-19 pandemic, Greenberg said.

The deluge of cancelled flights began with 2,300 worldwide cancellations on the Friday before Memorial Day, according to flight tracking website FlightAware. Saturday followed with 1,500 cancellations, followed by 1,640 cancellations on Sunday and another 1,644 on Monday. 

Cancellations continued on Tuesday, according to the flight tracker, with 1,231 flights canceled worldwide as of 4 p.m. Eastern time, 83 of which involved aircraft scheduled to fly to or from U.S. cities. 

Among major U.S. airlines, Delta Air Lines canceled the most flights during the holiday weekend. The carrier was forced to cancel more than 400 flights on Saturday and Sunday, and another 133 flights on Monday, according to FlightAware. 

Airfare quadruples in some markets

Airlines are expecting a jump in passengers this summer as travel restrictions have eased and pandemic fatigue overcomes fears of contracting COVID-19. Many forecasters believe the number of travelers will match or even surpass pre-pandemic levels. 

People who are only now booking travel for the summer are experiencing the sticker shock.

"It's not about the intensity, it's about the cost of an airline ticket," Greenberg said. "It's gone up about 7% every four days since the beginning of the year. So in many markets, the airfare has not just doubled, they've quadrupled."

Domestic airfares for summer are averaging more than $400 for a round trip, 45% higher than a year ago, according to travel-data firm Hopper.

The Associated Press contributed to this report.