The Southwest Airlines Pilots Association, which represents Southwest’s 9,000 pilots, has blamed the airline’s management and what it describes as poor planning for the trouble. The union raised alarms over the summer that pilots were being stretched thin by frequent reassignments that have led to longer work days and extended trips. Without changes, problems like this weekend’s could crop up again, union president Capt. Casey Murray said in an interview Monday.

“We’re going to see it next weekend or the holidays or whenever a thunderstorm pops up in Mexico,” he said…

In interviews, some Southwest pilots have said the constant prospect of being rerouted and uncertainty about the length of their trips could be contributing to pilots’ reluctance to pick up extra flights. The union said Monday evening that pilots did cover the majority of the trips that were left open due to staffing issues over the weekend.