Here's why:

* Cirques are bowl-shaped depressions carved into mountainsides by glaciers.

* The back wall of the cirque is often steep and cliff-like.

* The floor of the cirque is usually filled with a small lake or tarn.

* The lip of the cirque, where the glacier flowed out, is often a steep, rocky ridge.

This combination of features, with the steep back wall, the floor, and the lip, creates the armchair-like shape.