In the video, she is calling out saying 'Hello!' and you can hear a dog barking. When the dog appears, she tells it, "You'd better stay there, brother" and the dog makes its way towards her.
When you watch the video, watch how the dog behaves. It never growls at her. it never barks at her. It isn't even running full-force at her - something that would tell you an attack is imminent. The dog quietly comes towards her and when it reaches her, it attacks.
You can see her reach down with her left hand to try to grab the dog's collar and then shoots with her right hand. I'm guessing her left hand was in the way as she was shooting the dog because she tripped.
Thankfully, she didn't shoot off her finger and it doesn't appear that she was seriously injured as she was able to fire her service weapon so quickly to stop the attack.