Your mistake was using SATA to power your risers. SATA can only handle 55W and PCI-E ports needs to pull 75W.
Ok but why then risers V006C with cable 6pin >>> Sata works like harm? Made more than 20 rigs with them all powered through SATA and never had any problem, the same with those old red riser only with sata port, they work even with 1080ti.
If there were not enyough power they should not see the card, not burn them i think