Speaking of meltdown. 😁

Ouch, I'd say that it looks like the 12V rail passed some serious current through there, but then it'd have triggered the OCP on the PSU - so it probably shorted just enough to overheat and catch fire but not so much as to cause a shutdown.

I've had crappy PSUs with their molex pins protruding too much out of the plastic shell brush against the sides or bottom of the case, causing sparks and shutdowns/reboots, but no meltdowns/fires yet.

The other time it happened was on a brand new 3Tb hard drive which miraculously survived as the connectors on the drive didn't melt and it continued to work for about three years and then died.
I'm not sure if i ever want to use these adaptors ever again.

I see the water cooling didnt help with the fire 😁

I don't see any arcing on the end of the sata connector so I bet it internally started arcing and then lit up. Maybe a bad plastic molding on the sata connector side?!

Was the cable pulled tight in the when installed? could of cause extra pressure to the molding.

It probably wasn't enough of a short to trip the Over current protect on the PSU to shutdown.

old post

It looks like there is/was a bit of electrical tape on the adaptor. Was it a home made adaptor? Of the hundreds of Molex to SATA adaptors I've used over the years, only a few have ever failed and none have burned up. The few that failed were either pulled apart in there packages, or fell apart (at the Molex end) when being plugged in. I'd say that what you got is a really rare, odd ball occurrence if it was a commercially bought adaptor.