Glass, Michalske and Bunker said, has microscopic , shallow surface fractures. These fractures cause little trouble if there is no water present. Because of surface tension, water cannot enter the microscopic surface cracks - if there is no stress present. Under stress, water is forced in and moves to the very tips of the cracks. The new layer, called a hydroxyl, blocks further contact with water and prevents formation of thicker layers.
Hydroxyls are both softer and weaker than the glass.
Stress, concentrated at the crack tips, causes the hydroxyl layer to fracture, exposing the glass, again, to chemical change. A new hydroxyl layer forms, and the process repeats in a process called propagation. Without stress, a hydroxyl layer forms very slowly. My copy of Sinkankas is the 1964 edition and is very good. And, yes, I did take my conclusions to Terry Michalske.