I don't have an answer for WHY but I had the same problem with my 504
wagon. I fixed it by adjusting the shoes to not be so close to the
drum. I know that doesn't explain what the problem is but it did
help. The only time the problem resurfaced was when the car sat for
a long enough time and a light coating of rust covered the drum
surface.
I don't have the wagon anymore but I suspect it could have something
to do with the load leveling hydraulic compensator. It is supposed
to keep a correct proportion of braking power between front/back even
during heavy loading by way of a lever hooked to the rear suspension.
When travelling in reverse the rear raises quite a bit and thus
maybe that little hydraulic gadget is kicking in incorrectly. This
is just a guess, of course....
Senen.