[Not a pilot.] To a layman, it seems possible that the co-pilot seeing the altitude dropping rapidly and not sure which other instruments to trust, concluded that the plane was in a nose dive. The crew of 1979's TWA 841 (the nose dive over Michigan) knew immediately that they needed to pull back. I'm not saying that these examples are too similar, but such a mistake is something I could understand to be a result of two or so minutes of profound confusion.
As a layman I wonder what hints the center stick was giving Captain Gibson in 1979 compared to those offered by the side stick in 2009.