It is not a common known stat. You made it up. If you didn’t, simply provide the list of leaders on the NFL career negative rushing yard list. Because if there’s a number one, then there has to be a number two.
Now, I know you didn’t make it up, but it’s basically just an urban myth that has no evidence backing it up. Because I’m pretty sure if there is a leader in this imaginary category, it would be Emmitt himself. Not because Emmitt was prone to go backward more/less than average, but because he carried the ball over 4400 times in his career - by far the most in NFL history and almost 1400 times more than Sanders.