Remember that at busy airports, especially ones in NYC, the controllers are aware of departing aircraft before they even push back. They assign them a wheels up time, and probably times for leaving the ramp area as well. If you listen to the company traffic feed at JFK, you'll often hear (JBU) pilots calling ops to double check on when they're allowed to call ground for taxi. The airline's dispatcher also can play around with who goes when, say if they have a gate delay for one flight, they can swap slots with an aircraft that's ready now.