Bolts are most likely tightened with a torque wrench or a gun that is set to a torque spec. Over tightening a bolt is as bad as a loose bolt. I speculate these passed QA from Boeing because they might have been correctly torqued to the spec. What happens in field is hard to understand. One possibility is vicinity to the engine can cause extreme vibrations, these can make them loose. Other possibility is the maintenance side of things - maybe a badly calibrated torque wrench could be the reason. Mechanical systems are not inherently immutable.