This appears to be the coincidence of several situations.
It is unclear why the man was handcuffed, but clearly the police were on edge.
The man failed to secure his dog properly before speaking with the police.
The dog was instinctively protecting his owner, but did not appear immediately threatening.
It appears that the policeman fired his gun as a result of his own agitation, rather than any genuinely perceived threat from the dog.
In my view, the shooting was not justified, although I suspect the police will back their officer who has only to say that he decided his safety was threatened.
Personally, I think defence with mace, a baton or a taser would all have been preferable options to shooting. Shooting guns tends to make other people feel like shooting their guns, so it should always be a very last option, and in this case, it was certainly not.