It's probably from airframe age. Too many cycles on it, and poor or nonexistent inspections. There will probably be an investigation because this isn't the first time a 737 has lost a roof panel due to stress on the airframe from pressurization. It happened to Aloha Airlines in 1988. One flight attendant was blown from the aircraft.
The FAA found that the aircraft was old, not well taken care of and not well inspected (there were actually visible cracks that were overlooked before the flight). They slapped a Huge Fine on Aloha and the company is currently no more. With the recent incident, it sounds like one of the 737's Safety measures Tripped, a "Controlled Area Breakaway Zone" designed to rupture before the rest of the structure did. Except on the Aloha plane it was so old that when this happened the rest went with it. The Southwest plane was lucky that this didn't happen.