Nothing to do with PSU it self nothing to do with how much current system draws. You can produce flame with AA battery as well.
Looks like just faulty soldering or faulty connectors. Loss of proper connection causes open circuit > resistance closing to infinity thus current without other path will try to flow trough and heat up at point where resistance is high causing connections to catch fire. Might as well be just your own fault if you did not connect wires properly but I would go with cheap connectors used that doesn't fit together too well.
They are using seven 6 pin connectors to power 8 GPUs + aux power. This is bound to end in this type of failure eventually. Technically there should be 16 6 pin connectors to play it safe. They used the bare minimum. (don't forget a normal 400/500 GPU is pulling half its power from the PCIE slot/riser and another half from the 6 pin).
no they are using 5 and splitting 2 into 4
so
1 single 18 gauge
1single 18 gauge
1 single 18 gauge
then a single 18 gauge that splits into 2 single 18gauge
then a single 18 gauge that splits into 2 single 18gauge
and every photo of every burnt setup shows the melt starts by the split cables.
I mention that this psu should not be used long term more then one time in my review and that I went to a rosewill tokamak 1500 watt with no issues