No explanation was announced, but NetsDaily quoted a spokesman for the team and arena as saying, "There were a very minimal amount of people coming early except for the big games, when we will continue to open doors 90 minutes before tip-off."
That suggests some variability, and bolsters suspicions that this is a response to lesser fan interest as well as an effort to save money at a time when arena finances are hurting. As NetsDaily noted, some fear it might foster delays in getting through security.
Policies around the NBA vary. The New York Knicks have a 60-minute door policy, perhaps reflecting the options people have to entertain themselves before the game, while the Chicago Bulls and Los Angeles Lakers have a 90-minute door policy.