Iced up wings. It was reported there were icing conditions between 12K and 20K feet of altitude. That aircraft was in the zone.
That is what brought one down in NY a few years ago.
Commercial aircraft all have de-icing and anti-icing equipment on the wings, engine air inlets, and windscreen. Significant icing doesn't appear instantaneously and automagically, it accumulates over a period of time, giving pilots opportunity to find a warmer or drier altitude before it becomes a problem.