Many deck collapses are caused by failure at connections, typically the ledger fasteners. But this case was different.
Images in a recent NBC video report appear to show that in this case, the deck joists ran parallel to the house and were fastened to beams running perpendicular to the house at intervals of 8 or 10 feet. However, the beams and ledger did not give way; instead, it appears that the end-nails attaching the joists to the beams failed. It looks like no joists hangers had been used.
Deck collapses often happen when decks are eccentrically loaded, as when a large number of people gather in a small area to take a photo. The resulting point loads put extra strain on connections, especially on older, weather-worn decks.