If you were trying to back up and park and the car next to you open the door and you accidentally hit it whose fault is it?

The moving vehicle is At Fault. It would be the same answer if your were driving forward and the same thing happened. It is every drivers sole responsibility to be aware of everything around his/her vehicle whenever it is moving. My dad gave me 4 little Words to remember whenever operating a moving vehicle: "Leave Yourself An Out". It has two meanings:

# Always leave yourself an 'escape path' to get out of a nasty situation - this means never following to close to the car ahead and never pulling up bumper to bumper at a stoplight ... in either case, you virtually have 'no way out'.

# Always have the thought that nobody else can see you - you are invisible - this means you have to watch out for the other driver at ALL times, and there are no exceptions to that rule - requires defensive AND offensive driving technique constantly. I've not caused any accidents in over 44 years of driving using these simple four Words.