It's to keep the sides of the wagon from opening. There's a lot of weight pushing outwards from the gravel and it puts a lot of stress on the hinges. That's why the straps are placed between the hinges, to unload some of the weight on them. So it's not a "fail" ;)
This door wasn't made for this dog, that's for sure! When I look a little closer, I'm not sure I know many dogs that would fit through this door. Do y ...