That's frightening, I hope there wasn't a serious injury.
To be fair the fault there lies with a badly constructed unsafe jump, not the concept of arena eventing. What worries me most isn't even that the jump isn't weighted down but rather that the fence is so flimsy he's got a front leg through & trapped so is attached to the fence. It looks like it's built of plastic pipes, those things are sharp if broken & certainly not solid enough to be used in that way. If the fence had been made of wood it's unlikely the leg would have gone through that way & then it tipping, though not ideal, would be less of a danger.