Very interesting video that shows a clear picture of how the plug is held in place. Some interesting spots in the video, 8:44 shows how the upper two locking bolts lock the roller pins into the upper door tracks, and 10:00 show the lower two locking bolts onto the sliding hinge posts. 13:10 shows how the door blew cleanly off, with very little damage to the roller pins and lower hinge posts. Some comments say the same plug have been used on the 737-900 which didn't have this issue.
My armchair speculative guess: there are only 4 bolts effectively holding the door onto the roller pins and lower hinges. Somehow I don't think it's a simple case of someone forgetting to tighten the bolts; since they're using castle nuts, forgetting to torque them down would leave behind extra cotter pins. Those bolts look positively tiny, probably no more than M12 diameter, and are subject to intense shearing forces. In the case of the upper roller pin with locking bolt, they are effectively two cylinders perpendicular and on top of each other, which causes extreme compressive forces to be concentrated on one tiny spot on the bolt. My guess is they cheaped out and switched to an inappropriate/softer bolt, which sheared and/or corroded. If one shears, the load quickly spreads to the remaining 3 bolts which all also shear off. This theory would still jive with OPs article about UA; "loose bolts" may not necessarily mean "untightened nuts and bad QA", but rather, signs that the locking bolts are all beginning to bend or shear.