Offensive, defensive or what?

[finnref]

Worked two high school games with a Division 2 ref last week. He was lead and becoming the new trail on change of possession. Offensive player takes outlet pass, turns quickly, and begins quick dribble up court but runs right up the back of a defensive player who was slowly running down court to assume position. Offensive player knocks defensive player to the floor. Div 2 ref whistles defensive foul and explains to the shocked player that he was not in guarding position facing the dribbler so he gets the foul.

Discussed this with another college ref and he essentially agreed with the call. My position is that the defensive player was at that spot on the floor and is entitled to it by rule and may not be displaced. I compared this to the "over the Back", push foul that we frequently call. He insisted that the call cannot be offensive but either a defense foul or preferably a no call if you can get away with it.

Any opinions on this ?? Thanks