I saw that vid on an Australian site yesterday, it's awful and I hope the girl wasn't badly hurt.
He may have aimed at the first girl who said fuck you, but because of speed hit the second. Or he may have deliberately aimed ahead of the second bike knowing to make allowance for the speed. I don't think it was set up and the girl sounded in a lot of pain.
He seems to have acted in anger because they didn't take kindly to his heckling, wether he intended to cause serious harm or not, he intended for that ball to hit one of them, why throw it at them otherwise?
There's been some disgusting comments about the girl, she should have been polite, she should have been happy a boy wants to talk to her. I hope she's ok and I hope someone has a word with that boy about it's inappropriate to react with violence when a girl says no, politely or otherwise. If it it was an adult holding the camera they should be ashamed of themselves too for not going over and seeing if she's ok.