I am not convinced that it is a bogus trend. The frequency of reports has increased. I’ve been watching this trend since I first heard of it—also in 2011 - and this phenomenon—as well as the larger set of of unprovoked non-robbery related attacks by blacks on non-blacks—is increasing.
(Note that Hispanics and Souteast Asians also join the fun in one or more of the attacks on Jewish folks in NYC.)
Part of the problem is that most media will not report black-on-white crime as such, refrerring to offenders as “youths” and “teens”. See here for many more examples.
If you want to see an example of the racial bias in news coverage, compare the coverage given this story to the Zimmerman affair and note the races involved.
ETA: More details on media coverage of violence:
The Knockout Game: Racial Violence and the Conspicuous Silence of the Media
ETA2: Thomas Sowell, himself African-American, commenting on this:
This is virtually a verbatim quote something my father told me 30+ years ago.