At a news conference in Houston, Southwest spokesman Jay McVay said cancellations snowballed as storm systems moved across the country, leaving flight crews and planes out of place.
In normal times, this can allow Southwest to operate more flights over a given 24-hour period than other carriers, said Scott Meyerowitz, executive editor of The Points Guy travel site.
Casey Murray, president of the Southwest Airlines Pilots Association, said Southwest had not adequately prepared flights for the storm, which hampered ground transportation and maintenance vehicles.
Adding to the difficulty: Large swaths of Southwest crews remain in contract negotiations with the carrier.
Southwest Airlines did not immediately respond to questions about ongoing contract negotiations with its crews.