Eck, that's magic physics. The ball cannot move forward at all if pitched behind him, period. At worse it would be perfectly still. It's a fumble, defense's ball.

...

Thus fumble.

Of course if someone wants to get out the rukebook and find if it spells out the difference between "passing to a person behind you" vs "The direction the ball is effectively travelling" in regards to this scenario, that's fine with me, I'm not that motivated.