Amazon software, a too-aggressive workforce drawdown, and simulator shortages are among the causes speculated for Delta’s cancellation of more than 500 flights over the US Thanksgiving holiday weekend.

After about 1,800 Delta pilots accepted early retirement packages and another 1,700 approved a deal not to fly, the airline has “a smaller pool of pilots qualified and ready to fly in the fleets that are seeing an increased demand over this holiday,” said the the Air Line Pilots Association.

Under a cost-cutting agreement last week, pilots who would have been furloughed will receive pay for 30 hours a month, though they won’t have to fly. Delta can also reduce pilots’ minimum guaranteed work hours, which results in lower pay, in exchange for foregoing further furloughs until January 2022.

Many of the cancellations reportedly involved Airbus A320 and Boeing 737 aircraft, and Delta may be experiencing training backlog because of insufficient simulator time for required re-qualification.

Another possible glitch was an outage in Amazon Web Services (AWS), used for crew scheduling.

“A number of factors have pressured our ability to timely staff several dozen scheduled flights,” Delta said in a statement. “We apologize to our customers for any inconvenience these flight cancellations have caused.”

Only two flights were canceled as a result of the staffing shortages, according to an airline spokesperson.

American Airlines is reportedly facing a cabin crew shortage after furloughing 8,000 flight attendants in October when government support funds expired.