Buck fights Spitz because Spitz attempts to steal his burrow. Spitz thinks he is superior but Buck decides he has gone too far this time. This scene is suspenseful because you didn't know whether it was going to be to the death this time.
Dolly goes mad and chases after Buck, so Francois called for Buck to run past him so he could kill Dolly because she had gone mad with rabies. This scene is suspenseful because you didn't know whether Dolly was going to catch Buck or not.
The whole pack chases after a snow rabbit but Spitz decides he wants to take a short cut, then he kills the snow rabbit Buck so badly wanted to kill. This scene is suspenseful because Buck and the pack were chasing this rabbit and Jack London made it seem like Buck was going to get it, and the Spitz got it, which was an act of war in Buck's eyes.