Why does my car leak fluid only when I reverse park?

When parking, or at any slow speed, the power steering sees the highest pressures that it assists with. I find it unusual that it only happens in reverse.
However, one important thing is to not overfill the reservior as over-filling it can cause seals to fail.
If it is loosing enough that you have to top it off regularly, first remeber that depending on whether or not you have just been driving or not will affect the level. Try to always check it in either cold or hot states which will give you a better idea of how much fluid it is losing.
Note that one the power steering reservior the are different lines not only for low and full but for cool/full and hot/full.
I am assuming you see a puddle of luid under the car when you park in reverse. Check the reservior cap gasket as it may be bad and when you reveres it ends up on the ground while pulling in forward may leak fluid into the engine bay where it is captured and does not leave an apparent puddle. If it is leaking in reverese it is almost definately leaking in any low speed or stopped situation. Look for fluid in the ening bay and on the suspension or any area behind the reservior where it may be going. Try to trace the path of the leak. If you do not see any fluid, then the leak is in a lower hoes or in the steering rack itself.
Try getting some Lucas Brothers, "Stop Leak" as it will help. It will not stop bad leaks, but it will buy you some sime as a leak near or in the steering rack is not only letting fluids out, but contaminates in.