Who is at fault when 2 vehicles are parked side by and one vehicle hits the other while backing out?

It depends on the specific circumstances of the accident. Generally speaking, the driver who was backing out is usually considered at fault because they have a duty to be aware of their surroundings and to yield to other vehicles. However, if the other vehicle was parked illegally or in a way that obstructed the view of the driver who was backing out, then that driver may be partially or fully at fault. Additionally, if either driver was driving under the influence of alcohol or drugs, or was otherwise negligent, then they may also be held liable for the accident.