You had too much draw on your board. Do you really have 100cm(3 feet) long risers?
If they are actually that long, your GPUs would pull too hard and that's why you had a burnout.
The length of the USB cable linking the riser to the "shortie board" that connects to the motherboard DOES NOT AFFECT THE POWER DRAW.
That is NOT the issue at all.
Backwards riser - possible but that more likely would cause the system to not power up at all.
I'm more inclined to think there is an issue with the "add second PSU" adapter - one reason I like the B250 Mining Pro 19-slot MB as it has that part built into the MB and common-links their grounds the easy AND HARD TO MESS UP way.