He didn't announce arrest. And he was not required to do so, say some.
Technically, when the subject was pulled over he was under arrest-a traffic stop is considered a non-custodial arrest by the courts. Once he refused to sign the citation, he was technically telling the cop that he wanted to see a judge right now.
Once the cop started giving commands, he was taking the subjects liberty, leading to a formal custodial arrest. Custodial arrest involves contact and physically taking custody, hence the “Put your hands behind the back” As the cuffs are applied, the suspect is told they are under arrest under most states laws and procedures.
I will admit the officer could have handled the situation better, with some better verbage when the subject refused to sign. (If you refuse to sign I am required by law to take you into custody, etc.) He could have used better tactics, but I will not do a MMQB on what he did-I was not there.
The subject was completely out of line trying to run a trial on the side of the road. If he had problems, he needed to tell the judge, not the cop.
Depending on his department policies and state laws procedurally, he appears to be doing what was authorized.