Check your risers they be burned.
are you sure that "burned raiser" is not an urban legend?
bad molex contacts is something I encounter very often, but actually burned raiser is something that I haven't seen yet, but then maybe I'm just that good

Yeah I had a burned molex (peripheral cables)... It actually burned out on the PSU itself and damaged one of my outputs. Have just ordered a new cable from corsair and used a different output... What could have caused this? Can I do anything to prevent it? Could a claim be made vs corsair for warranty on my 1200w PSU?
Are you using SATA to 4-pin/6-pin power adapters? If so then most likely that's the reason. Most of the bundled SATA to 6-pin/4-pin converters are of cr@p quality.
They are not crimped but are molded. At high temperatures the molds basically starts to melt and can short the wires.
Avoid any SATA -> xyz power adapters. Use molex splitters. They might look flimsy but are constructed better than any other molded SATA connectors.
Also avoid loading more than higher number of gpus (using splitters) on single molex cable.
p.s. The SATA cables that come bundled with PSU's are crimped and not molded so they're good.