It was an inferno inside those towers, Instinct took over and it was either be burned alive or jump.
In this moment of hysteria, they probably felt that there MUST be some large cushion or fire rescue airpad of some sort since it had been a long time. They may not have been able to see the street or could not tell if they had nets/airpads down there. With the fire and heat getting to levels where they were near death, they jumped hoping that at the bottom, they will be saved by airpads.