You can see from the video that the handbrake wasn't on . Leaving it off is usually what happens when a driver gets hit by his own truck. It's one of those split seconds mistakes that can cost someone dearly ( either the driver , or padestrians), its especially easy when you're in and out of the cab and you're rushing.
It could also be mechanical failure , especially in freezing conditions. Just over two weeks ago , on a frosty morning, the handbrake warning sensor on the truck that I was driving , lit up several times , saying that the handbrake had come off( it had). Fortunately I was