Big mistake from City in the second half IMO. Sitting back in a derby is never a good idea - especially against Utd. You know the other team a) Will do absolutely anything/everything to get back into the game, and/or b) have the quality to do so.
Tbh - City's tactics of "sit back and rough them up" have largely failed in general. Utd have got their heads down and played football, City have been more interested in it being a derby. 2 fairly lucky goals against the run of play but as soon as Utd were level, City's heads just dropped.