I have seen this several times, in Superstar Mode (it probably also happens in other modes too). When I'm the QB and I pass to a player, and he runs forward, there are times when he gets pushed backw...
Ideas
I just saw it again. It was 3rd and 1. My running back (computer controlled running back because I'm in Superstar mode and I only control the QB) ran and got enough for a first down, but he was hit by three players and forced backwards 5 yards! I saw his legs were making the running animation, and I knew what was coming...and, just as I expected, not only did it NOT GIVE ME THE FIRST DOWN, but it marked me as 4th and 5. I LOST 4 YARDS because the other team forced my running back backwards. Again, that's not how it would be ruled in a real-life game. In real-life, they would have ruled forward progress and given me the first down.