Can anyone tell me what the hell is happening to my pc?

>>365313
Shitty injection molded parts that have a defect and get too hot and melt. Usually this happens with molex to SATA adapters but it can happen with any adapter. Replace it and don't cheap out.

>>365314
nothing wrong with my power supply?

>>365313
looks like that particular connection was not properly touching one side to the other, creating heat, enough to melt the plastic around it
do those cables lead to an adapter for the graphics card? this is an accident waiting to happen. either replace the molex to pci express converter, or the psu, as soon as possible (unless you want to start a fire while playing some games) and make sure its connected all the way before closing the case