The long stretch of temperate weather, major arrests of gang leaders sparking turf wars, and changes to police tactics are mostly responsible. Of course other social and political issues are always there pushing things along too.
Still the per capita murder rate doesn't crack the top 10 in the country even with the recent increase. If you want to see a place with a truly shocking murder rate New Orleans would be where to go where, with all respect to the General, troops did not solve the murder problem.
To really tackle the problem the city needs more cops, decriminalization of drugs, and to address the social problems inside the black community that contribute heavily.