Crust floats on the mantle. Mountains are areas where the crust is thick and it goes down into the mantle further than less thick areas. As the weight from the top is removed, that can push up areas from down below.

We tend to think of mountains as stable things with the constant stress underneath, rather than a layer of congealed fat on top of a warm stove (or, a lava lamp). But the latter is a better model for geologic behaviour over long periods of time.