Stop bolts
Thinking further about the way the upper and lower stop bolts work. The lower stop bolts will be much stronger than the upper ones because of the way they are fitted. The upper bolts attached to the guide at their ends. The guide roller then contacts the bolt at its mid point which is its weakest point, being furthest from where it is attached to the guide. The lower bolts again attach at their ends to the green hinge bracket. The force on those is applied by the shaft which should be applied along the length of the bolt and under extreme stress would be split between each end of the bolt at its strongest point.
Assuming this is correct, would it be fair to say that the lower bolts are the important ones and the upper ones are just a secondary insurance? Now if the lower bolts were missing the upper bolts would have to do all the work. There must be some play in the fitting of the upper bolts such that the guide roller can move a little bit between the top of the guide and the bolt. Without that it would be very hard to insert the lock bolt.
If the lower bolts are missing the plug door would be able to bounce between the top of the guide and the lock bolts with any aircraft movement. Eventually would that be enough to cause the two bolts to fail? If they did fail then that could also damage the guide near the fixing holes for the bolt.
This would explain why the failure took some time to happen. From what everyone else has said, it is hard to see how the plug would have popped out after a few months if no bolts were present. Unless they were loose of course.