Navigators on the GRAIL team wanted to rule out the remote possibility that either of the probes would crash near a lunar heritage site. They tweaked the probes' trajectories to impact in a lonely site near the moon's north pole, far from any Apollo or Soviet landers. NASA's moon history is in the spotlight lately because of new missions planned there; the space agency doesn't want any of its historic artifacts interfered with. Along with the historic preservation motivations, there are scientific reasons for this--there are some trash bags and other waste astronauts left behind, and it might be useful to study those someday to see how any microbes may have fared on the freezing, barren lunar surface. New landings could stir up moon dust and interfere with those measurements.