Imagine a tree on the edge of a brink. The soil erodes under influence of the wind and water, and the roots of tree become exposed. The binding between the tree and the soil becomes very weak and a strong wind can easily uproot the tree.
The same process occurs with the metal surface when it wears. The atoms easily break away and the destruction of the surface begins. Moreover, the whole groups of atoms can break away under extreme loads. This results in a catastrophic wear.