I went to the Daily Mail to see if they had an article on the shooting. Of course they do.
https://www.dailymail.co.uk/news/article-12878885/PICTURED-Adorable-four-year-old-boy-shot-dead-road-rage-driver-10-days-Christmas-Gunman-chased-parents-car-started-blasting-family-shopping.html
I read the article. It does not explain how one gets behind a car by cutting it off.