Without knowing all the facts, I will only comment on the generic scenario of LE going into a situation that may be harmful to them. That being, much like I mentioned in the Bundy thread, why do it? If you believe that your life or well-being is in imminent jeopardy, but no other life is in jeopardy and there is no other emergency, why would you proceed? Why not back off, and in this instance, if it was indeed simply an eviction notice, call out to the homeowner, call animal control, or just wait.
If he had already proceeded into the yard without first seeing the dog and feared for his safety, why pull the gun first? For the last probably ten years, every local, state, and fed uniformed cop that I have dealt with at work has had a pistol on one hip and a taser on the other, plus often a baton and pepper spray. Why not use one of them first?
Just this afternoon, my dog and me were riding around on the farm when a dog I didn't recognize came barreling out of my cousin's orange orchard barking like crazy and chasing us. I had a .45 on my hip, and a "dog stick" (because it's not the first time this has ever happened) on the rack of the ATV. I ended up using neither because the dog calmed down when I said "hi buddy" in a friendly voice and started talking to him. Had that not worked though, I would have gone for the noggin' knocker before I pulled a gun.
Perhaps more facts will come to light here, but I would still question going for the gun first for a single dog, when almost certainly other weapons were at the officer's disposal.