The cyclist was arguing with the driver when the lights went green. You can here someone toot their horn because he was delaying the traffic behind from moving.
The horn is from the traffic coming n the other direction (you can tell by the way the sound passes rather than just stays consistent as a horn from behind would), probably because the car, in attempting to edge around the three bikes had also crossed into the lane for oncoming traffic. They all set off within two seconds of the light going green, if the horn is coming from behind it is because of unreasonable impatience, not because of delays.