Brady's video really helps understanding of the mechanism.
Speculation: If all four bolts are missing, the odd thumped on landing with minimal if any pressurisation could allow the springs to push up the plug. Whether it was one thumped landing or a few, the door ended up off the pads and let go the next time there was enough pressure.
Whatever the cause, the interior panel prevented crew and/or maintenance from seeing the plug was coming loose.
Most likely crews will in future promptly descend and evacuate seats if there's noise from the plug.
The inside panel needs to have view ports to allow verification the plug is properly seated. A paint stripe across the plug and adjacent fuse would show up during external inspection.