I myself have wondered the exact same thing and even asked many sports enthusiasts the same thing..Running backs constantly lower their helmets and constantly committ head to head contact..I guess it might have something to do with a running back or wide receiver having only one use of an arm considering they are carrying the rock and lack of time to make a conscious decision to do it..Then you see a lead blocker either being a running back,fullback, or receiver lower their head and going straight for the defensive players helmet..