God.... that's pretty horrible. Poor guys...
Something like this happened on a much smaller scale in my Northern Ontario home town years before to a Beaver float plane. The cargo, which is stored directly behind the cockpit, broke loose during take off, sliding back to the tail section so the plane nosed up much like that jet did, and when it stalled, the plane keeled over. In that case, the cargo then slid forward to crush the pilot and his passenger before driving the plane nose down into the lake. Because of the air trapped inside, the tail section remained vertical, sticking out of the water much like a buoy.
In that case, the transportation safety guys determined it was pilot error, as he was responsible for securing the cargo.