Or they don't understand you're about to stop and backup, so they pull behind you too close, preventing you from reversing. Even if they realize and want to backup to give you space, there may be someone behind them already.
This is something that only works if everyone is taught and if everyone agrees to behave well. This might be more feasible in regions where parallel parking is common and drivers are accustomed to stopping way back to allow the stopped car in front to reverse.