It happens from 2 things:
Too many plug/unplug cycles, even the best available pins for PCIe plugs are only rated for 25 cycles. El-Cheapo's as few as 5 cycles and whatever plating was on the pins is gone. Once the plating is damaged things go down hill very quickly. If the miner or PSU are used, who knows how many times the plugs have been cycled...
Using aluminum or too thin wire for the PCIe cables. The wire itself is part of what removes heat from the connectors. If it's only 18ga then the pins WILL become warm, the plating will begin to oxidize and eventually fail.
Thank you for the concise explanation. So new pins and better wires should prevent this from happening.
I thought maybe it's a hardware glitch or done type of malfunction from power supply or pmu.