I used to see that as well. When dogs pack up they are dangerous regardless of breed. Of course some breeds are inherently more aggressive than others. But I’d see the local usually friendly dogs pack up when let loose. They would get more aggressive. Fortunately no one got attacked.
There was a Chihuahua in my old neighborhood. It constantly attacked my 149 p0und Briard. It was kinds funny! Pandy would pick it up by it’s neck and throw it 15/20 feet, Still the rat came back for more!.