It's both. The cable, as designed, has practically zero headroom for safety. Under ideal circumstances with all wires and pins working optimally, it can delivery slightly over 600W of power. Any failed wire or pin will cut that maximum value drastically, which is what is happening. Coupled with this is that nVIdia removed the per-pin safety feature that would automatically shutdown wires in the event of a failure and thus reduce the maximum power draw of the card. Now when a wire / pin fails, the card continues to pull the same power but the remaining wires have to handle the excess load which is above what they were rated for.
Contrast this to the six and eight pin PCIe power connectors which have a safe limit 2.2x higher then the rated limit. One or two pins could fail and the cable would experience no problems.