Air travel was a mess over the holidays — in the last 10 days of December, 30,000 flights were canceled. While every airline was affected, one stood out: Southwest, which over the past few decades has transformed how Americans fly, melted down. In the last 10 days of the year, it canceled as many flights as it had done in the previous 10 months. So what went wrong? Guest: Niraj Chokshi, a business reporter for The New York Times.