Actually there was a reason: there was an ongoing feud between the dog’s owner and the local police because he has made complaints against the police.
I can’t watch the video and haven’t watched it but it needs to be spread around because I think this behavior is an example of how society is becoming more stressed. That stress is affecting how people behave including the police.
The police are becoming too defensive and they are overreacting. It is isn’t just in America. In Britain it is probably worse. You can’t demonstrate in Britain anymore. The police just corral you (called “kettling”) and then arrest you for nothing.