Who is at fault if another car suddenly went into reverse and hit you on the left side rear door when were attempting a U turn?

The fault of the accident will depend on the exact circumstances of the incident and the traffic laws in the jurisdiction where it occurred. In general, the driver who is making the U-turn has a duty of care to yield to other vehicles on the road, and to ensure that it is safe to do so before proceeding with the turn. However, if the other car was reversing in an unexpected or unsafe manner, they may also be held partially or wholly liable for the accident. Factors such as the speed and location of both vehicles, as well as any signals or warnings that were given, will also be taken into account when determining fault. It is advisable to seek legal advice or consult with the local traffic authorities if you are involved in an accident of this nature, as the specific laws and regulations can vary from one jurisdiction to another.