Speaking of crime analysis

What's useful in all types of analysis, not just crime, is looking at outliers to figure out what is happening or not and why.  So in the face of the general drop in crime and murders, experienced by most cities across the country, why is it that Philadelphia and Houston are experiencing significant increases in murder, even if the rate of other crimes is dropping?  See "Rate of Killings Rises 38 Percent in Chicago in 2012" from the New York Times.

Gangs are one possibility.