Is this a foul?

[New AZ Ref]

I am a first year official in Arizona.

Working an 8th grade tackle league using Federation rules.

Working as Linejudge.

Play goes to far side.

Right in front of me, about 20 yards across the field from the play, A1 hits B1 in back and knocks him down. I flag it and go to talk to the White Hat after the play. I thought it would either be a block in the back or unnecessary roughness. Penalized it as a block in the back. White hat pulled me aside at half-time and said that it shouldn't have been a foul because A gained no advantage from the action.

Would do you think?