Its not falling, I am the one who head butts it.It seems that because of the low angle, the break mechanism could not handle the weight of the door.
Can you put a stick to avoid the door failing on you.
Otherwise, in general I put my groceries on top of my back seat, because I find it more easy than opening the rear hatch.
Also I have some seat covers, and I put a black towel on top of the itemsto hide them from the outside.