This shaking, known as tremulation, is a form of communication between male tree frogs.
The males are tremulating to establish which is the dominant male. They’re claiming territory for their ‘calling area’ where they spend the night calling for a female mate.
Sometimes, the shaking leads to wrestling among males… and maybe even more shaking, until the loser retreats.
See the video at National Geographic Link -Thanks, Marilyn Terrell!