It appeared the tear was to the plane's skin, its outer protective metal, not the jet's internal structure, an aviation expert told ABC News.
The jet was built in 1996, according to an FAA database, making it 15 years old, not ancient by aviation standards.
However, Southwest planes get a big workout because they do so many takeoffs and landings a day, which puts stress on the metal fuselage.
Two years ago, a hole opened up in the fuselage of another Southwest plane, a jet also 15 years old. Investigators blamed fatigue cracks, and Boeing ordered extra inspections for its 737s.
In 1998, a flight attendant died after being sucked out of a hole in the ceiling of an Aloha Airlines flight over Hawaii after cracks caused part of the ceiling on that Boeing 737 to peel open.
Hours after their emergency landing in Yuma, Southwest Flight 812's passengers arrived in Sacramento aboard a reserve plane.
Ashley Main was delighted that her boyfriend was safe and sound. She'd been waiting for hours, ever since receiving a frightening text message.
"He sent me a text when he was on the ground and told me there was a hole in the ceiling of the plane and told me he was all right and did assure me that -- but I don't believe him," she said. "My heart sank. I have a fear of flying anyway and it scares me to think he could be lost to me."