Kind of improbable, but not much else left that is consistent with old exposure of the copper and passing the Megger testing:
The wires had been damaged for a long time, although probably not broken or even stripped that badly. As long as they were not touching anything and the conduit was dry they could still have passed on the Megger, depending of course on the voltage it applied.
Then when the VFD was connected the pulsed DC, with or without standing wave voltage enhancement, caused an arc and breakdown. Possibly the motor starting current even caused movement of the conductors as a result of the magnetic field. (The megger testing would not have involved current).
Or the normal current caused rapid heating and fusing of one or more damaged wires, freeing the loose end to make hard contact to the raceway.
Only one wire with a fault required for that.
Finally, when the wires were pulled out extreme damage occurred to multiple wires, obscuring the less obvious damage that led to the failure when the wires were first actually used with the VFD.
