I recall sitting in extreme rear seating on more than one occasion on a DC-8-73 stretch. As you go through high altitude turbulence, you could actually see the "torquing" motion, looking up the aisle towards the front of the aircraft, as you were being shifted from side to side. I always thought it to be the fact they added fuselage plugs to lengthen the aircraft to add more seats, without changing anything else from the shorter length version...
TD