Here's why this simile works:
* Leaves are delicate: Leaves are easily moved by even a slight breeze.
* Leaves tremble in the wind: When wind blows, leaves shake and flutter wildly.
* Shaking and trembling are similar: Both actions involve rapid, uncontrolled movements.
Therefore, saying someone is "shaking like a leaf" emphasizes the intensity and helplessness of their trembling. It suggests that they are completely overwhelmed by the situation and have lost control of their body.