I'm not defending the officer. What I am saying is that the video does not show what kind of agression the dog was showing as it was in the frame for about two seconds before it disappears. That dog's tail was not swinging wildly from side to side like when he greets you at the door. An erect tail, such as in the video, is a sign of alertness. Not necessarily agressiveness, mind you, just that he's on alert. As he jumps out of the vehicle, that alert tail will bounce.
The dog and the cop were out of the picture and at the last instant the cop backed into the camera frame with what "appears" to be the dog lunging at him. Unfortunate as it is, it becomes a he said/she said determination on the part of any judge.