If there is an accident involving a car backing out of a parking space and a car driving past in a parking lot, is the driver backing always at fault?

I notice more and more people racing past when I’m backing out of a parking space, even when I’m a foot or more already backed out.

Today a woman at Walmart slammed on her breaks directly behind my (backing) car, honked her horn, and made exasperated gestures. What kind of sense does that make? If I’d hit her would it have been my fault, even though she was stopped in a driving lane?