Rebounding Foul or Not?

[Blindolbat]

Was doing a B league rec game with some fairly competitive adult men. I was the L and a 3 pt shot went up from the left side corner of the court. An offensive player rushed in unblocked from the right side jumping up for the rebound. His momentum took him onto a defensive player on the left hand side of the key that was attempting to box out another offensive player. The player that was landed on had no idea the rebounder was about to land on him as he was moving and trying to box out the other offensive player.
Should a foul have been called on the rebounding offensive player for landing on that player?