likely  small crack…   electricity is lazy..  it always finds the shortest route to ground .

could be manuf. defect showingup years after  or to much torque on a bolt  caused a crack for humidity to sneek in            who knows…..

Not that uncommon to see this.  Perhaps the plastic casing broke down a bit, allowing the secondary to “short circuit” to ground through there.  Or, perhaps the plug/gap/open secondary/high resistance, causing the secondary current/voltage to look for an easier path to ground.

All that being said, the opposite is not always true, especially on those external coils.  Just because there are cracks in the external plastic case does not mean it is arcing to ground.  I have seen many of these coils with cracks, that don’t leak, and work just fine.  I’m guessing the crack is only in an outer plastic case, and that there is another inner casing that is “OK”.  The only way to confirm this would be doing the secondary probe test with a coil tester.    Seeing those “pin holes” is usually a good sign of leakage though.