There were vehicles in the plane, if they were not properly secured they could have rolled backward as it climbed out, if the load shifts forward or backward it changes the center of gravity, when the center of gravity changes the stall speed increases, so a pilot could be flying at a safe speed above his stall speed and if the cargo rolled forward or backward the stall speed would increase and he wouldn't even get an indication that he was about to enter a stall. Centers of gravity are important calculations that every private pilot must be able to calculate, most (if not all) aircraft manuals have charts showing stall speed increases in knots with every inch of change in center of gravity.
I was going into a short strip cut off the top of a mountain once with nothing but my wife and myself sitting in the two front seats, I planned an approach circling a log pond 50' over the runway 5 knots over stall speed so as soon as I touched down I could stand on the brakes so I wouldn't run off the end of the short runway, when I crossed the threshold, gear and full flaps down, I cut the power and the manifold pressure, the plane dropped the 50' with a very hard landing. I checked the manual when I got back, had the wife been in one of the center or rear seats I would have been fine, but with both of us in the front I had a forward center of gravity increasing the stall speed (a Cessna turbo 310), if I had more passengers in the plane I also would have been fine.
