While backing up your car, the gear jumps out of reversing.
Car Problem Diagnosis
Transmission is very complex so it is hard to determine the exact cause of the problem unless the transmission is inspected on actual, however we can determined the most likely caused of the problem based on scenario.
There are two parts of a transmission that has something to do with the transmission staying in reverse gear, namely the shift linkage and the tension spring on the shift rail. When those parts are faulty such as worn or out of adjustment linkage or weak spring tension it can cause the transmission to pop out of gear while reversing.
However this is only a possibility, there might be other cause which can only be determined by a professional by way of actual inspection of the transmission.
Possible Cause of Transmission Jump Out of Gear on Reverse
Faulty or worn out shift linkage or Weak tension spring on shift rail.
All About Auto Advice
If your car is still covered by auto warranty then you can claim for a necessary repair on your car dealerships at no cost, however if it is beyond the auto warranty bring you car to a reputable auto repair shop that specialized in auto transmission job and ask the mechanic to check for the faulty or worn shift linkage or weak shift rail spring tension, if the problem will not solved then another possible cause can be determined by thorough inspection of the transmission and it will cost you a lot.