I assume this was an NG because that's all SWA flies right now.
The areas I'm aware of where large crown skin cracks have occurred are underneath the vertical stabilizer fairings where a rubbing condition can create damage and a stress concentration (I believe this is under an AD-required inspection program, but couldn't quickly find the AD - possibly I'm confusing an issue on another model), at chem-milled steps in the crown skin panels (AD 2013-08-16), and in the areas around some STC-installed inflight entertainment system antenna cutouts (AD 2014-03-19), which have typically been installed in the middle part of the fuselage where crown skin loads are high. I'm assuming this was not a lap splice issue because the report said the crack was in the crown skin. There is no lap joint at the crown. I have not seen anything specific yet related to this report, but the fact that the pressurization system couldn't keep up and it was reported as a 12 inch crack makes me think it was a chem-mill pocket crack that turned the corner of the pocket and opened a flap, which is what the design is intended to do if a crack occurs. I'm guessing, though.