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).