In this scene the two dogs, Buck and Spitz are growling a each other because they end up being in the spot at the moment. This scene is suspenseful because it looks like Buck and Spitz might fight and it is hard to tell who could win.
In this scene Francois saves Buck from the dog Dolly, who had rabies. This is suspenseful because Buck could have died from the crazy dog.
In this scene Buck kills Spitz because Spitz started the fight and now Buck kills him for the dominance. This is suspenseful because Buck could have died from the fight.