The driver who is backing up is not always at fault, but these drivers are often at fault. This is because the other driver often has the right of way. For example, if the crash happened in a parking lot, the driver in the feeder lane or thoroughfare had the right of way. This is stated in Ontario’s Fault Determination Rules (Rule 19).
The statements above may come as a surprise to you because many people assume fault is shared between both drivers because both cars were in motion. This is sometimes referred to as the 50-50 myth. While there are cases where drivers share fault, one driver often bears most of the fault.