They were the two robbers who had escaped from the prison. The robbers noticed that Billy’s dad recognised them, and decided to kill the witness. But suddenly, Billy’s dog smashed a rifle and destroyed the explosives, this way getting robbers’ attention. They got frightened and while cleaning the mess they got into a fight.
At that very moment, the dog ran to his owner and tried to rescue him. While the dog was trying to rip the rope, the robbers noticed them and tried to stop Fluffy.