Why did the dogs attack Boxer?

The dogs attacked Boxer at Napoleon's command because he questioned Snowball's removal.

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. The pigs' ears were bleeding, the dogs had tasted blood, and for a few moments they appeared to go quite mad. To the amazement of everybody, three of them flung themselves upon Boxer. Boxer saw them coming and put out his great hoof, caught a dog in mid-air, and pinned him to the ground. The dog shrieked for mercy and the other two fled with their tails between their legs. Boxer looked at Napoleon to know whether he should crush the dog to death or let it go. Napoleon appeared to change countenance, and sharply ordered Boxer to let the dog go, whereat Boxer lifted his hoof, and the dog slunk away, bruised and howling.