Why are some secondary consumers carnivores, while others are omnivores

A trophic level of an organism shows its position in a food chain.
1. Producers, such as plants and algae, produce their own food and thus are the base of every food chain.
2. Primary consumers eat producers. They are herbivores that eat plants, for example, a deer, a rabbit, a grasshopper.
3. Secondary consumers eat primary consumers. They eat herbivores, so they are carnivores, but could also be omnivores. Carnivores are meat-eaters and omnivores are both plant-eaters and meat-eaters. 
4. The highest level is tertiary consumers that eat secondary consumers.