The first pic should give you an idea on what I'm talking about. As it sits in the picture, with nothing in the drop bag under the front bench, the cooler can rock forward (i.e. step near the latches and the bottom can rock slightly forward). If the drop bag is full of firm stuff the cooler won't rock at all in any direction.
Occasionally, I run two coolers and in this arrangement neither cooler can rock. They support each other. I have a piece of 1/2" plywood that I can put ontop of the straps and set the cooler on if I'm running pretty much empty. That also stops the whole shebang from rocking as well. I think it's just the roughness of the ply wood on the straps that keeps it from sliding.