What would cause this?

Sorry guys, no pics, this was around 7-8 years ago I had this happen to me, and I just remembered it and thought to ask. So we used to have an old Athlon XP computer, that my parents built themselves. They use an Antec case that came with one of the infamous Antec Smartpower blue power supplies. I think around 2006 when we'd had it for 4 years or so, if I'm remembering correctly, they gave it to me when they got a new one.



If I'm remembering the specs correctly, it had an Athlon XP +2100, 1GB of DDR RAM, an MSI motherboard, and a Radeon 9700 graphics card.



Not long after I got it, I started having issues with the USB ports. They would randomly stop responding, and after a while would work fine again. Eventually one day it just refused to proceed past POST. When I pulled it apart, I discovered why. Some of the pins on the 20 pin connector were melted. No, the plastic was actually melted. The pins were blackened as well. When I looked it up, they were for the +5 rail.



It ended up frying the motherboard, but only so that it wouldn't POST. It still would turn on, and I could still get into the BIOS. But it wouldn't boot, I had to go back to an old Pentium III system until I got a replacement motherboard for it.





Thinking about it now, and knowing what I do now about power supplies, I'm still confused as to what could have caused the plastic to literally melt and what looks like catch fire, as the metal pins were actually blackened. A motherboard short maybe? Or could the power supply itself have been to blame? I know those old Smart powers were notorious for bad Fuhjyuu caps, and other problems, but nothing I can think of would cause that.



Thoughts?

What would cause this?