They had to cancel the majority of their flights for almost a week to get back on schedule.
The problem is Southwest's software systems. They are out of date, and designed for a much smaller airline. As Southwest grew, they failed to scale up their tools and systems. As a result, their planes and customers where stuck at the airports, while their crews were stuck waiting for assignments.
They had a matching / assignment problem.
Back in the middle 90's, I consulted with United Airlines on their software system that determined which flights to cancel, in order to minimize disruption. It was complex software that had been specially built for them. Southwest, I've read, used some generic, off the shelf package that they customized.
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