Flatbed tow truck brake failure on incline - GIF

[Rangi]

I am sure that there is a rock behind the wheel. As the truck rolls back it pushes the rock back as well. At the end of the video there is a large scrap mark and the large rock.

[schuylergrace]

I think the scrape mark and the sound you hear as the truck rolls back are from that bar that extends down to the ground from the back of the truck.