There's some slack in every manual transmission, and when you shut a manual off in gear (like you should), that slack gets 'taken up' as the vehicle tries to roll away because of gravity. It's hard to explain. Try this: park it in gear on a level surface. It will roll backwards like you say, but it will also roll the same distance forward. There's just the slightest bit of play in the transmission, like in all manual vehicles. You'll notice it won't bind up like this when you engage the clutch to start it.