Well first of all, the Zotac tested your linked review is a different card with a completely different HSF than the OP's. Secondly, see all those cards running hotter than the GTX 460? They didn't spontaneously combust in Anandtech's overclocked rig running furmark.
Plus, the OP said his rig was idling. The cards weren't doing any work at all, and neither was the southbridge. The event you are describing is so unlikely as to have a practically zero chance of occurring. The OP had a short or surge in the lower PCIe socket which caused a brief, intense electrical fire before his rig shut down. The card may still be to blame somehow, but the cards simply did not spontaneously combust due to overheating.
This is not against you but just for the sake of possibilities. In theory, at idle, nothing should be hot, and nothing should ignite within a computer even if it is overclocked. Clearly, it did. Even if there was an electrical short, the PSU should shut down in milliseconds. The damage was clear that it can't be done with just a simple short unless the PSU shut down or it actually did but something else was burning after it has shut down.
Note that the mobo and psu can still function, indicating that the short can not be sever or it will kill both. Also, we should be able to locate the source if it is indeed a several short by locating the pins and sever damage around the pins that caused the short as the temperature of the pin must be higher than its surrounding. We are not able to locate such pin.
The damage seems to occur somewhere between the lower part of the 2 video cards. Knowing the cards are populated in slot 1 and 3, making socket 2 the possible area where shorts occurred. There are no questions about the possibility of a short from that slot, but can't be a short that cause the damage on the card because a) there is only one portion of the cover that is on top of the slot is burned, b) the burn of the cover is far sever than the slot itself, c) the cover itself is non-conductive, meaning it can't be the domain of the short. d) We should be able to locate the domain of the short as it should severly damaged, more so than the cover, yet either it isn't on the picture, or not visible through the picture.
If sever electrical short didn't occur, than what caused the damage? Fire was obvious as there weree ashes and smoke, but nothing seemed to be the fuel of the fire. There are no fuel, but there was a fire. How can that be possible? I checked those stickers but there ain't burned, but more like covered by ashes.
Like all fuel, its mass will reduce. In simple terms, it will be consumed. Nothing in those pictures were consumed other than the plastic cover as we can see the ash on the back of the 2nd card as well as the ashes at the 3rd PCIe slot. If it wasn't the fuel, than what kept burning it into ashes?