Here is CBS's coverage of the event:
A deli worker told CBS News New York that Hodge was ordering a sandwich when a woman at the counter got angry and began arguing with him over who was first in line to order. According to the deli worker, in that instant, the woman pulled out a knife and stabbed Hodge in the stomach. USPS confirmed Hodge was a letter carrier assigned to Manhattan.
It appears to have started over a dispute over who was first in line. It's up to you to decide if that's a rational reason to attack someone with a knife, regardless of your sexuality / gender identity.
https://www.cbsnews.com/newyork/news/usps-worker-stabbed-to-death-in-harlem/