In the beginning of chapter three the rivalry between Buck and Spitz intensifies, Spitz never passes up the chance to growl at Buck or bully him. Until this moment, Buck never engaged in any violent activity with Spitz, in fact he avoided fights. But when Buck saw that Spitz was sitting in his nest, he attacked Spitz.
Buck had just finished running for his life from Dolly who had gone crazy and spontaneously chased after him aggressively. Because Buck was exhausted Spitz found this as the perfect opportunity to attack Buck. The unsuspecting Buck suffered severely ripped flesh that was torn to the bone, while Spitz suffered a whipping from Francois.