A backwards pass should be a loose ball play, because the ball is not in player possession. This is the key to the difference between a muff and a fumble. You can muff a snap but you cannot fumble a snap. Any play has a loose ball play (the snap), then a running play, and then another loose ball play (forward pass or kick), followed by one or more running plays. Those running plays can be interspersed by other loose ball plays (backward passes).