Blocking from behind

[eventnyc]

Yesterday I had a penalty flag on a play. I work the Umpire position on a varsity crew. This is my first varsity year. There was a run to the left outside. I turned in the direction of the play to watch for any infractions out in front of the runner. To my immediate left, a lineman came off the line and was running in the direction of the play. He was at least fifteen yards from the ball and only a few yards from the LOS. Out of nowhere, one of the linebackers came from behind the lineman and plowed right through him. He hit him square in the back and knocked him forward to the ground. I threw the flag and when the play was over reported the infraction to my referee. When I told him that I had a block in the back against the defense, he looked at me quizzically and said "against the defense?" I repeated the infraction and we enforced the penalty. At halftime we discussed the flag. The referee said that I should have called a personal foul for unnecessary roughness. What's the difference on this play?