Im not a rigger or instructor or anything...
But couldn't it be that when your backflying your less exposed to the relative wind then when your sitflying(or headdown)?
My theory is that the wind will push the bridal more towards the rig (the wind will make it stick to the back of the rig because the force is at another angle) as opposed to sitflying which will make the bridal fly away from the rig.
Of course you shouldn't do stuff with your rig it isn't designed for(if it isnt designed for other uses then bellyflying you shouldn't use it for anything else then bellyflying) but I was just wondering if there could be a little truth to my theory , and if not please tell me why, thanks
(I made a schematic illustration of what I mean) Parachute gear garage sale at :http://www.usedparachutes.eu