Apparently in NYC it is the implicit responsibility of the second person in line at a red light to immediately honk their horn when the light turns green so as to remind the first person in line that it is their turn to go. Having been the second person previously, subsequent drivers (usually numbers 3 thru 5 or so) are often concerned that this person may not be aware of this responsibility or is simply in need of assistance. So they take it upon themselves to also gently remind those in front of them of their responsibility by utilizing their horns. Who knew NYC was so full of concerned and considerate driver??