From what I've read, the nightclub is in a predominantly black neighbourhood, so I can imagine just two reasons for this shooting:
- Gangland violence (most likely)
- Racism (far less likely, perhaps the shooter was "avenging" cops who've been murdered recently)
Let's explore our imaginations for a moment:
- Dislike for one or more of the people participating in the social event.
- Dislike for someone running the club
- Dislike for the parents of one or more of the children participating in the social event.
- Rejection by one of the girls in the event, whom the shooter was infatuated with
- A tweet or other statement made by a third party about the club which convinced the shooter that the club was bad
- The shooter was thrown out of the club or not allowed in
- Someone at the club who was not present but who the shooter assumed would be present embarrassed the shooter in some way
- The shooter is actually the owner of a rival club who was losing business to this club
- The club was a front for drug dealings which were infringing on the shooter's turf.
- The shooter had been driven mad by a neon sign on the club which was shining on the shooter's apartment keeping him/her awake at night.
- The shooter decided to end his/her life and wanted to make the news and picked the nearest target he/she could find.
I'm sure there are others.