The problem comes from manufactures no following the specs. A 20 pin mobo connecter should be able to supply 75w total to all the PCIe slots, 24 pin is 150w total. Reference GPU cards are designed to pull no more than 25w from the PCIe slot.
What

I thought they pulled 75W from the slot.
They aren't "supposed" to. But some do. And one reason why you get burned up mobo connections. Most common cause is a loose connection though. That causes a reduction in the cross section available to the current, causing overheating, and in worst case senarios, arcing, sparking, and fires.
