Defense Offsides?

[RookieDude]

Basketball Official here trying to get a rule and reasoning behind a play I saw last night.

A buddy of mine was doing a big H.S. Varsity football game last night, so I thought I would go and check it out.

Buddy was the visiting side linesman.
The visiting team was on defense. The defense only had 10 players on the field...so a visiting team player ran out on the field to make it eleven.
The only problem was that when the player left the sideline, he was now on the offensive side of the ball. It was close, but in my buddy's judgement (and mine) the player did not get across the line of scrimmage before the offensive team snapped the ball. Thus, a defensiive player was on the wrong side of the ball, even though he was running toward the defensive side.
Buddy threw a flag...discussion with Referee...picked up flag.
Why?