He is disturbed by his mother’s behavior toward the man.
Explanation:
The story says that " The cub could not understand. He was appalled. The awe of man rushed over him again. His instinct had been true. His mother verified it. She, too, rendered submission to the man-animals." This means that the cub, (Assumed to be White Fang) Is disturbed by his mother's behavior toward the man.
