Auto Insurance Claims: Backing up incident in a parking lot, parking lot accident, rear view mirror

Question
I was in an accident in a parking lot where I began to back out of my space and after I started to back out another vehicle on the other side of the parking lot begins to back up.  Once I see her backing up I stopped and honked my horn since there wasn't enough time to put the car into gear and move forward safely.

My car was not moving and sustained no damage, however, the other vehicle sustained damage to her trunk when she backed into my bumper.  When she got out she said she looked to the right and to the left but she didn't look in her rear view mirrors.  I had a passenger in the car with me that knows I stopped.

Out of the blue I get a call from my insurance company about a claim.  Also at the scene she asked me if I'd make a claim... I said "I don't know" because I hadn't really had a chance to make sure my bumper wasn't bent.  Later I checked it and the car was fine.  

She had a hard time communicating, but we exchanged information at the time.  

What I'd like to know is am I at fault?  I started to back up first and stopped and honked my horn in the other vehicle backed right up into my vehicle because she wasn't looking in her rear view mirror?  

I think her husband was angry and wants to get us to pay for the fact that his wife damaged their brand new Lexus on the back of my late model Expedition.

I am in the state of California and the parking lot is a private lot. The police wouldn't come out for a parking lot accident and I have one witness to corroborate my side of the story, the other vehicle has no witness.
Any thoughts?

Answer
Hi Steve,

Parking lot accident can be some of the most difficult to determine liability, especially when both parties are backing up. You should strongly deny that it was your fault. Provide your insurance company with your witness information so that they can take a statement from them. However, since this person was in your vehicle, their statement will no carry as much weight and an independent unbiased witness.  Your insurance company will likely eventually make the decision to accept 50% liability. This is how it usually goes when both parties are backing up.

I hope this helps
Richard Hixenbaugh