Apparently about 10-15 years ago, after a spate of engine separations on B737-200's, the bolts were changed by Boeing to a slightly revised design.
Nationwide apparently failed to ensure that their aircraft that they operate now had these revised bolts. They are very expensive, and in typical Nationwide fashion, they didn't install them. The CAA did warn them to comply with Boeing AD's, and they didn't listen...Can't say they weren't warned.