Because the person filming it was not filming the event. They were filming security footage of the event that was being displayed on the screen. The screen appeared static, and the camera happened to capture the truck doing a hard right turn, and wasn't tracking that truck in any way. The person filming the security footage being played was tracking the truck, presumably to show us all what happened.
So why was the camera following that particular truck?
Because the person filming it was not filming the event. They were filming security footage of the event that was being displayed on the screen. The screen appeared static, and the camera happened to capture the truck doing a hard right turn, and wasn't tracking that truck in any way. The person filming the security footage being played was tracking the truck, presumably to show us all what happened.
Thx for clearing that up Fren.