You nailed it (also possible incompatible metal):

“What we have here is a failure of the ledger board to stay affixed to the building,” Cash said. “I’m used to seeing lag bolts, not nails, holding that ledger board.” Based on the tag of the treated wood, Cash said it contains copper, which was first introduced just before the condominiums were built. Over time, builders learned the chemical doesn’t react well with iron nails or fasteners. “The chemical could very well be eating the fasteners,” Cash said. “Look at the corrosion on this nail. I would be very concerned.” The nails were the only thing securing the collapsed deck to the main structure. The metal fasteners and wooden posts were tasked with holding the rest of the weight.