In Never Cry Wolf by Farley Mowat, if wolves were not the cause of the decline in caribou and deer, then what was?

First, we should note that wolves and large herbivores such as deer and caribou have been coexisting with quite stable populations for many centuries and thus there are no grounds for arguing that wolves are responsible for population declines of caribou. Instead, wolves tend to prey on weak, sickly individuals, culling the herds. Even more important, wolves serve to keep herbivore populations in check. In areas where wolves have been exterminated, deer populations spiral out of control, leading to overgrazing, which eventually causes starvation and mass deaths. Large predators keep populations of herbivores at sustainable levels.
The two main causes of the decline in caribou found by Mowat were disease and human predation. Human hunters are quite efficient at killing caribou and much of the effort to claim bounties on wolves and reduce their numbers was instigated by hunters, especially sport hunters, who wanted to have more freedom to hunt caribou with modern weapons (as opposed to the Inuit, who take only a small number of animals at a subsistence level) and to blame declining numbers on wolves rather than to take responsibility for their own actions.