Friendly biting incident I suppose. Here’s the full story from the video description:

LA MESA – February 4, 2013 – INFORMATION FROM RADIO TRAFFIC/MY OWN ACCOUNT: A police Sergeant is in the hospital tonight after a police K9 attacked him and bit him in the arm during an incident La Mesa PD was working around 8:30 PM. The officer was with La Mesa PD and the K9 was brought in by El Cajon PD. The original call was for a man who possibly attacked his brother with a hammer. La Mesa PD set up a command post for an uncooperative subject. They cleared the house, and then moved to the backyard. I was about to leave when I heard yelling and struggling in the backyard and ran up. Officers had to tackle the dog to the ground. The officers could be heard yelling “fuey!”, german for “stop”. The officer was able to walk down the street, leaving a very noticeable blood trail behind, to the command post and stood by for medics. Medics rushed in with lights and sirens and bandaged the injured officer up. Another officer drove the injured officer to Grossmont Hospital to be treated for his wound. The dog’s name was possibly Dax, as indicated by the El Cajon patrol car with the picture of a police dog with “Dax” next to it. I do not know why or how this happened, it may have simply been a mistaken identity.

It looks like the sergeant will be ok though.