The bullfrogs prey on the turtle's young, as well as newts, salamanders, and other smaller frogs— a cold-blooded killer in the literal sense
From the article:
“As bullfrog presence declined, we started to hear other native frogs call and see native salamanders walking around,” Woodruff said of their field work. “It’s nice to be able to go back to these sites and hear a chorus of native frogs calling again that previously would not have been heard.”
Bullfrogs are a particularly nefarious invasive species because they will eat basically anything that can fit into their mouth, and even as tadpoles they have pretty enormous mouths. You can see in the picture below the comparison of an older bullfrog tadpole and a juvenile pond turtle (as hatchlings they are even smaller, roughly the size of a quarter).
The enormous endeavor of removing bullfrogs from ponds has led to not only the return of the western pond turtle (California's only native turtle species), but also of native frogs and salamanders that were also being picked off by bullfrogs.
(Image Source)