For those who remember such things, something similar happened early in the RTX 4090's life cycle, when power connectors were melting and even catching fire, damaging the GPUs and power supplies.
After much investigation and many guesses from Nvidia and other testers, the 4090's power connector issues ended up being blamed on what was essentially user error; the 12VHPWR connectors were not being inserted all the way into the socket on the GPU or were being bent in a way that created stress on the connection, which caused the connectors to run hot and eventually burst into flames.
The PCI-SIG, the standards body responsible for the design of the new connector, claimed that the design of the 12VHPWR connector itself was sound and that any problems with it should be attributed to the manufacturers implementing the standard. Partly in response to the 4090 issues, the 12VHPWR connector was replaced by an updated standard called 12V-2x6, which uses the same cables and is pin-compatible with 12VHPWR, but which tweaked the connector to ensure that power is only actually delivered if the connectors are firmly seated. The RTX 50-series cards use the 12V-2x6 connector.
The 12VHPWR and 12V-2x6 connectors are both designed to solve a real problem: delivering hundreds of watts of power to high-end GPUs over a single cable rather than trying to fit multiple 8-pin power connectors onto these GPUs.
In theory, swapping two to four 8-pin connectors for a single 12V-2x6 or 12VHPWR connector cuts down on the amount of board space OEMs must reserve for these connectors in their designs and the number of cables that users have to snake through the inside of their gaming PCs.
But while Nvidia, Intel, AMD, Qualcomm, Arm, and other companies are all PCI-SIG members and all had a hand in the design of the new standards, Nvidia is the only GPU company to use the 12VHPWR and 12V-2x6 connectors in most of its GPUs.
AMD and Intel have continued to use the 8-pin power connector, and even some of Nvidia's partners have stuck with 8-pin connectors for lower-end, lower-power cards like the RTX 4060 and 4070 series.
Both reported 5090 incidents involved third-party cables, one from custom PC part manufacturer MODDIY and one included with an FSP power supply, rather than the first-party 8-pin adapter that Nvidia supplies with GeForce GPUs.
According to Ars Technica it's much too early to say whether these cables (or Nvidia, or the design of the connector, or the affected users) caused the problem or whether this was just a coincidence.