It's a crash, so don't do things that make your graphic card crash... buy better hardware, don't overdrive it, don't overheat it, get your graphics card manufacturer to make better drivers...
Obviously the reasons can be multiple. By hardware I guess you meant risers and accessories? The main hardware is pretty much standard (280xs, mobo gigabyte etc etc). As for overdriving, well all is stock. Heat, well cards are running at 70-71C. Perhaps the drivers would be my last resort by reinstalling, but I can only rely on those who produce it...AMD...
Could the crash also simply be a defective GPU?
I'd test each card on its own 1 at a time and mine with them for at least 30mins. This will find if there's a weak or defective card. If it finds nothing and all the cards check out its more than likely a bad psu or failing pci-e adapters on the motherboards.