I don't know,
I've read that plants have evolved over time,
More dangerous during X time period,
Less dangerous in Y time period.
Might be it was easier for some animals, to eat other animals,
Than to eat plants.
Or when certain plants were less nutritious,
Easier to eat the animals that were specialized in eating that plant.
Another thought is that suppose herbivores with no predators ate all the plants,
Well, was probably useful for the one's able to eat other animals.