Here is a very good article about the incident:

Police Officer Arrests Firefighter At Accident Scene In California : The Two-Way : NPR

I must point out that the article indicates that the CHP has jurisdiction on the scene, once they claim it. The article also says it was unclear whether they had done so. Regardless, however, the cop should at least have waited until after the incident was over before arresting any responder. It is not like the truck got moved any quicker because of the arrest, and he could have waited to prove his point about jurisdiction.