I was under the impression that this is more likely related to the reduction in lead in the air. At first glance that seems plausible. See murders by year[1] and the presence of lead[2], the timelines seem to match.

[1] https://en.wikipedia.org/wiki/Crime_in_New_York_City#Murders...

[2] https://www.forbes.com/sites/alexknapp/2013/01/03/how-lead-c...