The insider source is anonymous, but the article is by ace aerospace reporter Dominic Gates who broke the 737 rudder defect story. The article contains a very detailed description of how the door plug fits and what went wrong and how.
In a nutshell, fuselages arrive from the Spirit factory in Wichita with “a hideously high and very alarming number” of defects, so Spirit has a crew in Renton to fix all the problems. On this aircraft, problems were found on both door plugs, which Spirit repaired improperly. On the left door plug (the one that blew out) Spirit also damaged a seal trying to fix the previous problem. Boeing inspectors identified the improper repair job.
The article states internal documentation shows that Boeing and Spirit discussed whether replacing the seal required the door plug to be removed or just opened. Either scenario required removing the bolts. There should have been a Boeing inspector verifying and documenting the bolts were reinstalled correctly and that the seal had been replaced. But according to the article, no such documentation exists.