You attached the risers wrong
The data cables or USB cables were in the 1x pcie slots backwards
Been there done that.
You should read my thread here I have great 3 card riser free board builds
How is that possible to plug them in backwards?
I will photo.
see the standard 1x slots see the small slot with a little wall
https://i.imgur.com/HlJ3Zuw.jpgonly allows for this
but people using the 16x full slot
can do this which is fine
but they can do this. This is a short which can kill the card the riser the slot the mobo
the last pic in your post, it happened to me too but it didn't fry everything, it only fried an external pcie 1x connector(few pins) and sata to molex cable(cheap sata to molex came with the riser) I was using to feed electricity to the riser from the psu, first time in life I saw a cable turn red with heat and then it melted, I guess it pulled too much current... whole thing happened in about 5-10 sec when i realized the smell and cable melting, then I immediately turned the psu off.
everything else was fine psu, psu's cables and connectors, even the pcie 1x slot on the mobo was fine too and working. Note that my mobo was Gigabyte 990fxa-ud5