[...] 750Ti without power, steel risers burn through 12+ volts[...]
Of course they do... Those risers are not made for high power cards.
The very worst card for such risers, is the 750/750TI
without external power, as the card is entirely powered from PCIe, but even with externally powered cards, those risers are known to fry quite frequently.
Were you lucky enough to not get any damage beyond the risers themselves?