Another article says the police had numerous calls from people saying that someone was firing the gun at people, while the vehicle was being driven down the street.
Even on Independence day, that might be said to be disturbing the peace.
It appears that this is not that clear cut, because the only offence mentioned is a fireworks offence.
http://www.candgnews.com/news/july-4-panic-caused-antique-military-vehicle
Firing a fifty while driving down the street would draw attention and be disturbing the peace, even on the 4th of July.