Not enough small critters around to keep them fed?
There are several mechanisms for this.
One is the predators harvest prey beyond its ability to reproduce. This is a relatively uncommon event.
Another is the prey population breeds beyond the ability of the environment to provide feed. This happens fairly often. Food supplies are highly variable, so prey populations have enough food in some seasons and some years, but not in others.
Another is a large prey population contracts disease, which spreads through the prey population, killing a large amount, maybe even more than 50%.
Another contributor is predator populations increase with readily available prey. It doesn't seen that is the major controlling factor for prey populations, but it is a contributor when prey populations crash. Suddenly, there are way more predators than there are prey to sustain them.
The predator populations then become hungry and desperate, and explore otherwise ignored possible food sources.