Who is at fault in Washington state where a vehicle is backing out and is almost completely out when another car hit the car backing?

Fault here would likely be shared. On the one hand, the driver backing really has no legal right to the lane they are backing into

• you cannot cut someone off while you are backing up. But it seems

you have described a situation in which the individual backing was well into the backing process when someone came along, from quite a distance, and hit them. Hence the driver in the lane already probably had the last clear chance to avoid the accident but chose not to and would hence bear the majority of the fault.