Probably it was a tiny bit loose, once the one pin that was loose started heating up it was all over. The 980x has caused more of those to die than any other part. They haven't killed as many connectors as Fermi GPUs have, but the Fermis in SLI kill the ATX24P connector.
Modern stuff draws squat by comparison, which is why we don't see this much these days. Plus once the 980x came out motherboard manufacturers started putting extra connectors on (like the R3E, which was built specifically for the 980x).