This is relatively old news (search for IIHS crash test, and you'll find lots of threads about how the MDX did). Though newcomers may not have seen it, of course.
The MDX's head restraints aren't particularly well-designed. If you sit, you notice that they're a bit of a distance away from the back of your head and have an angle (the distance varies from person to person, of course). You can't adjust them. Better-designed headrests cradle your head closer and are adjustable.
The IIHS theory is that a better designed headrest helps prevent whiplash in a collision. Hence the marginal score for the MDX's headrests. That score is not based on an actual crash test, however, but by static measurements. Nevertheless, Acura should improve them.