Here's why:

* Isostasy is the concept of balance in the Earth's crust. It's like a giant seesaw, where the denser mantle acts as the fulcrum.

* When erosion removes material from the top of a mountain, the weight on the crust decreases. This causes the crust to rise upwards, like the seesaw tilting up on the lighter side.

* This upward movement is called isostatic rebound and is a slow process that occurs over long periods.

Essentially, the Earth's crust is constantly adjusting to changes in weight and density, striving to achieve a state of equilibrium. Erosion is one of the many factors that contribute to this adjustment.