When you roll a bike backwards it will pull the chain with it from the top, your crank will turn and on the bottom there is nothing guiding the chain. If in the big/big combination it will fall off on my bike and likely yours too. Moreover, when in this combination the pull of the rear derailleur will exacerbate the motion of the chain as it takes up the slack caused by the falling chain. In other words, once it starts falling you won't be able to stop it by moving the bike forward.