It's been said before, it never went away.
As an impressionable kid in the 90s, I heard it every time I went to the match. When you're a child, those fuckers are winning everything and City are shit, the only thing that would get you a response is if you called them "Munichs". The people, the noise, the pubs, the matchday culture is what most working class teenagers fall for. It follows then that they would pick up some bad habits. Habits which get drawn out when a spontaneous overflow of emotion - a hatred found on Derby day - overtakes any moral concerns.
That said, I think it's important to note that it's becoming increasingly detached from the actual event. United are a club which has no solid community or identity, most couldn't name a player from the crash, so they only act offended because they feel they should. Few of them actually give a fuck.
The subject actually bores me now, so I'll cut this post short. If you don't like it, don't say it.