Density is your friend, right? You asked why the officer who the dog was approaching didn’t fire, and, instead, the other officer fired? Since neither of us were present to see the context of what unfolded neither of us can know whether one officer had a clearer/safer firing position than the other. It is possible that the officer the dog was approaching didn’t have a safe line of fire, but the other one did. Is this really that difficult a supposition to consider? It is also possible the officer that fired was more aware of the dog and the potential threat than the officer the dog was approaching. But, as I said . . . Context is everything.
Good grief~