Perhaps it was a failed sale and the owner was taking it to the tip along with a load of other stuff, and happened to be on top of the 'stuff'. You were the lucky finder.
If the owner cared for it like we do our models, we would make sure it was either in a box, or at least nestled in the boot/somewhere in the space where it could not move about.
It is a shame the roof didn't fall out as well, but if you cannot trace an owner then the roof would not be difficult to make.