How and why did Buck start a mutiny among the dogs?

One night the dogs spot a rabbit, and they chase after it, joined by a nearby pack of fifty police dogs. Buck, leading the pack, rejoices in the hunt, all of his instincts overwhelming him. Spitz, cold and calculating, takes a different route to head off Buck before he can reach the rabbit. Spitz jumps upon the rabbit, and without thinking Buck leaps upon him. He knows that their time has come. This is a fight to the death. At first Spitz clearly betters Buck. After a few moments Buck is covered in blood, while Spitz is untouched. Spitz is an experienced fighter, but Buck possesses imagination. He tricks his rival, and is able to break both forelegs. Spitz is beaten. Buck knows that this is no place for mercy. He knocks Spitz onto his back and the sixty dogs rush in to finish him off.