I don't get the 6-pin splitted to feed risers.

The 6-pin or 8-pin cables that you plug into the GPUs only carry 12V but the riser also needs some of the 5V feed included in SATA/molex.
Edit: nevermind I see there are risers that only take 6-pins and stepping the 12V down.

Whatever the case might be, I think either the PSU is faulty or something is very wrong since I never heard people also frying stuff like CPU/RAM/disks with the motherboard.

I'd be curious to see some pictures.