The gray lady makes lots of guesses about why, both citing and distancing itself from the 'Ferguson Effect' being a cause.

Some officials say intense national scrutiny of the use of force by the police has made officers less aggressive and emboldened criminals, though many experts dispute that theory.
If not the 'Ferguson Effect', then what? This isn't about what's wrong. This is about why urban murders increased broadly across America just now? What's different today than last year?

An observation. Detroit isn't mentioned in the article. Not even once. This is remarkable. Urban murder article, and the word 'Detroit' is absent.

Full front-page article: http://www.nytimes.com/2015/09/01/us...ties.html?_r=0