why the growling of the dogs stop the pigs

Napoleon has asserted his control over the animals of the farm. Napoleon makes a strange whining noise and his guards, nine huge dogs that he stole when they were pups, attack his rival Snowball. Opposition against Napoleon is gone and he announces that Sunday meetings will be abolished and pigs, under his control, will make decisions. Some younger pigs begin to protest but the growls of the attack dogs stop them. They have seen what Napoleons enforcers are capable of.

The growling of the dogs have provided a sense of fear to the animals, so that when either Squealer or Napoleon says something they would not be rebutted.