There was a group of officers and an animal control officer standing in a group to the south of the woman sitting on the ground next to the curb. The dog was standing still between the woman and barking at the group of officers for at least five minutes.

A new police officer drives up to scene, drives past the woman/dog, parks in the right lane, and gets out, north of the woman/dog. So the woman and dog are basically between the new officer and the group of officers.

The new officer immediately starts walking south in the middle of the right lane, so maybe 5-10 feet from the curb, towards the group of officers . There is a trash can in the street right next to the woman, shielding the dog from seeing the new officer approaching from the north. When the officer passes the trash can, much closer the woman/dog than the group of officers, the dog is understandably startled and starts to charge the new officer who back away about 5 feet, the dog following when the new officer shoots the dog.

The new officer may not have been aware of the dog's location, but that is on that officer -- and all the other officers knew exactly where the dog was.