As to your question in the OP, of course men walking behind women is a tznius problem. That is the very reason why Halacha prohibits it. Halacha didn’t create the prohibition of men walking behind women for another reason. Rashi in Eiruvin 18b explains that a husband cannot even walk behind his wife.
http://www.theyeshivaworld.com/coffeeroom/topic/chivalry-038-yiddishkeit-a-foreign-concept