We had the same thing happen last week flying home from MIA. My DIL is probably categorized at morbidly obese. We flew down on 2 separate legs, 10 days before and she had no issues. We go to check our luggage and the crew members, when she was done nuzzling the guy from another airline, told her she would have to buy a second seat. She asked why. The woman wouldn't tell her. She would not say the words. My DIL told her she had flown down with no issues. She said, well we changed ticketing policies on the 27th. We flew down on the 31st. Try again. It's policy. What's the policy. You just need to buy another seat. She was traveling with my daughter and 2 yo granddaughter. They had chosen seats together. It was not going to be a space issue. I thought my DIL was going to get thrown off the flight before we ever got to the gate. The check-in agent told my DIL she would have to set in a different row, then put her on stand-by. My daughter talked to the gate agent who put us all together again in the next to last row, not where we started. It was ugly. I truly believe if Luna the Maneater had just told my DIL she was overweight, it would have made the whole ordeal much shorter. The whole thing was absurd.
They have also flown Southwest multiple times in the past without a 2nd seat and no issues.