the plastic part of the package is generally epoxy, injected into a mold containing the lead frame already welded to the chip. No glue is used to attach external pins to the internal lead frame, as the leads are part of that frame.

If there are multiple such failures, then they bear close examination, including detailed pre-assembly inspection, just to rule out manufacturing defects in the parts themselves, though I would suspect that's the likely cause.

The main place at which this type of package is "stressed" is when the leads are formed, which is done in a punch and die, which also trims off the outer portion of the lead frame, which holds the leads in place during packaging. As I said before, a bit of contamination could stretch or break the leads. A contaminant, e.g. a shred of lead-frame material could, but most likely won't, move around in the punch. You should be able to see that as a scuff on the leads before assembly.

RE