There were only 2 possible causes of the incident, either the bolts had been replaced with bolts made from an inappropriate material, so they broke, or the bolts had not been assembled correctly, and they became loose.
It appears that the latter is what happened.
Perhaps those who did the assembly of the doors at Boeing did not use the right kind of washers that are needed to prevent the unscrewing of the bolts, or they did not apply the correct torque to the bolts.
It is extremely surprising if such trivial errors can happen during the assembly of an aircraft.
It appears that the latter is what happened.
Perhaps those who did the assembly of the doors at Boeing did not use the right kind of washers that are needed to prevent the unscrewing of the bolts, or they did not apply the correct torque to the bolts.
It is extremely surprising if such trivial errors can happen during the assembly of an aircraft.