I think we will find out that the lift assist springs were out of adjustment. I would guess they are not designed to be providing a constant upward force on the door, which would put a constant force on that locking bolt.
It's right there in the name, lift assist spring. So, for example, if the plug weighs 100 lb, then the spring may provide 70 lb of upward force, and a mechanic opening the plug would only have to provide 30 lb of force to lift the plug high enough for the door stop fittings to clear the fuselage stop pads. This setup would cause a constant downward force (from the weight of the plug) keeping the upper guide roller in the door upper guide track fitting. And no constant force on the locking bolts
If the lift assist spring was adjusted in such a way that there was a constant force applied to the locking bolts, then I could see how after 3 months of vibrations the two bolts wear down and eventually fracture, which would allow the door open on its own.