You always have to be very, very careful when circling back to pick up your tubers or water skiers. You simply cannot whip the boat around speeding into those in the water. I know how tempting it is to think you are in complete control of the craft, but your aren't. Not in the water. Also, when getting anywhere near skiers in the water you have to put your prop in neutral!
I would speculate that boat operator circled around way too fast, did not put prop in neutral and then slammed into girls. Sad, needles accident. (Teens driving??)