I know that I am not the only one that runs into this issue. I set my focal point, get the focus that I want and go to take the shot, only to have my focus change when I re-compose. By going into your camera's custom functions, you can set up your camera so that it will only focus when you hit the star button with your thumb.
Since I only have a Canon, I can't be sure that this process will be the exact same for Nikons. As you can see in the images below, you select the second option in "1" in "Function IV: Operations/Others" and first option in "2".