Describe what happens after Napoleon accuses four pigs of being Snowball’s agents.
"Napoleon stood sternly surveying his audience; then he uttered a high-pitched whimper. Immediately the dogs bounded forward, seized four of the pigs by the ear and dragged them, squealing with pain and terror, to Napoleon's feet."
Then, in a frenzy of bloodletting, the "enforcer" dogs seemingly spontaneously turn on Boxer and attack him. Boxer is not as defenseless as the little pigs, though, and he crushes one of the dogs with his giant hooves. The rest run away scared of the big guy. Consequently, this might be a reason Napoleon is so keen to eliminate Boxer from the farm later on.
The dogs return, though, and Napoleon continues his trial. Either in a way that was predetermined, or for some reason in the heat of the moment, the little pigs confess to the crime of supporting Snowball, after which: "When they had finished their confession, the dogs promptly tore their throats out..."
This action was probably designed to show the animals that Napoleon was serious. Why he chose his fellow pigs to enact the terror on, well, I am not sure. It might have been a way for him to let the others know that he would go to any lengths to "protect animal farm," even at the expense of the seemingly "top of the food chain" pigs.