There are people that already asked to end this off topic discussion, so I don't want to keep it going and going.

I will try to be clear so we can move on....
 as others noted in the thread-
 6pin PCIe to 8 pin, its added ground path. Look at the pin out on those connectors. 6pin = three +12V and -12v, rated for 75watt. This are typically heavier gauge than the motherboard 24pin.  If you look closely at the mother board connector pin out, there are two +12v wires yet only one single -12v to connect the loop. You have to realize, it should be obvious, there is only one negative path from the motherboard plug to the psu.  Remember now, the 75watt rated 6 pin PCIe has 3 pins for -12v which are more robust on top of that.  Am 8pin is not much different than the 6, it's the same three +12v with the extra pins being added -12v to accommodate the extra load.
The motherboard, it has one single -12v pin. Should be obvious, it's not made robust. Significantly less than the PCIe 6pin.

The -12v would have to carry the entire 12v load, on this connector, it is the weakest point.  If there is an excessive 12v load, on this plug the sole -12v pin would be the weak point, most likely place of failure. Heating up metal, plastic, air, and gases in and around that particular pin, lowering resistance and creating the perfect condition for an arc.  Since current is returning thru the negative pathway, it is (-) remember this. The short occurred not thru the +3v but from the +3v which had found a path straight to the ground (-12v pin).  The 3v was straight from the psu and is supposed to enter the board but instead had found a path straight back thru the -12v beside it.  

You would not expect the -12v pin to explode, it will carry all the load the metal will allow, the heat melts plastic before the copper and metal pins, the heated gases ionizing, then the 3v finds its destiny shorting thru the -12v.  

I hope you get it, I don't want to be redundant.  Too high current heats up and melts things, if it's not way past the specified load then this will not be sudden or instantaneous. a short to ground will create  a much mote dramatic blast. a sudden explosion.  The -12v pin would not explode, current wouldn't flow out of it to other pins as it don't work like that.  The +3v found its way to ground.  

+3v and +3v will not short.  The issue is not the power, it's drawing that much power thru connectors, traces, pad, etc that were designed for less current.  It's important to keep paths in the circuit insulated from one another.  These paths were designed and rated, intended for the load.  This is why there is a max specified load, the components, pathways, connectors, traces, pads, etc all have an intend range.

A short circuit occurs when positive finds a straight path to ground.  If there were no other paths or traces nearby, a larger than intend load could only act as a fuse, zip and the connection is gone. 
In reality, there are plenty of pathways on a circuit board. They are really really close to each other.  So, what happens... the extra load causes excessive heat, lowering resistance until an actual short occurs between positive and negative. Positive straight path to ground.

The picture, you see the middle pin above the two +3v pins?  It is the -12v.  Usually, the ground is far away from the positive in a lot of plugs.  Had the -12v been by the +12 in this case, there would have been a much larger arc blast.  See, the -12v on this plug was the heat that lowered resistance, melting and the the +3v lanes had a path straight to ground. 

+3v to +3v would do nothing.  +3v straight to negative....you see the result