For loose bolts, incorrect assembly is by far the most likely cause.

It can be a design problem if the bolts and associated parts like washers have been substituted recently or if the assembly instructions have been changed recently, e.g. by specifying a different torque. If any such engineering change has happened recently, then that would be the likely culprit.

A resonance problem of the aircraft body as supposed by another poster seems extremely unlikely as that would have required significant recent changes to the aircraft body, which did not happen.

The same part was used on the 737-900 for quite a while and didn't (as far as we've heard, at least) have any similar issues there.

And you seem to be assuming that the problem is in the design and not the assembly...

The latter seems more likely.

You are both missing the 'faulty process' and 'faulty communications' options.

And:

- "bad parts from vendor" - "bad programming on torque wrench from MRP" - "insufficient training on process" - "tooling was programmed correctly but bad sensors" - "lube mislabeled for 3 days"

This is absolutely not some simple thing. This is why people don't take us seriously as engineers.