Right! But, if you zoom out, it's not exactly too wild. Ground squirrels have voles beat in size and they have habitat overlap with both living underground. Additionally, dietary shifts give animals the flexibly to respond to changes in the environment plus foraging risks and opportunities. It's been known since the early 2000's that squirrels can supplement some of their diet with insects or bird eggs/baby birds. Knowing this, it starts to make a lot more sense ground squirrels would hunt voles, as the environment is rapidly changing, they're known to already hunt a bit, and voles could be their annoying neighbor.