In a community the number of predator-prey can be limiting factors for the growth of the populations in the environment. As the prey population decreases, the predator population begins to decrease as well, this event can be seen in the graph of the question in two points of the community curve we can see what can be highlighted as the introduction of a new predator to the environment of this community.
Therefore, in point A and C a new predator starts to be part of this food chain and community, happening a fall in the population size.