Why was she on the ledge to begin with?
Having read the backstory, it seems that: she was on the ledge with two other friends and all agreed to jump from the bridge. One of her friends had just jumped and she agreed to jump with the other friend, who I believe is the same friend who pushed her. She was hesitating, so the friend that pushed her starting counting down because others were waiting to jump. When the girl who was pushed balked, the pusher pushed when countdown hit zero.