I'd offer the reason head and arm and shoulder hits are more common may be paralled to rioting where the upper body is something you can still strike on someone standing a few rows back. Its quite possible to hit an opponents head in the 2nd row (even from your own 2nd row) if you are in tight formation, however obviously his body is protected largely by the opponents body in front of him. Again you often see riots, the front lines are crushed and engaging each other, and the next ranks back take overhand pot shots at anyone's head they can see sticking up. Particularly dangerous in this scenario are things like long handled axes or picks as their ''rightangled'' shape means they can still fall on heads more than a row back.