Clearly, if you hit a stopped vehicle with yours then you are at fault, regardless of whether you were going forward or backward. Similarly, anyone backing up has an obligation to use the utmost care.
Here is a fact you should know . Any accident in a parking lot is not covered by any insurance , only wrecks on public roads . Insurers do cover accidents that occur in private parking lots. The police however will not respond nor make a report unless there are injuries. The person who was backing up will most likely be the "at fault" driver