It could well have been a manufacturing defect that was not found during overhaul.
Even the CMM procedures do not cover all the inspections that would need to be carried out. For example the CMM might state 'carry out Magnetic Particle Inspection of shock strut IAW ASTM 1441", it would then be up to the MRO or overhaul shop to write their own in house inspection procedure based on the guidelines of the ASTM.