As the glaciers retreated they left behind steep rock walls. These valley walls were smoothed by the passing ice and rock. A few millenia of soil accumulation and tree growth has done little to wear at the underlying rock creating ideal conditions for landslides.
The thin soils with sheer rock faces underneath need only a little push in the right direction to peel away, rock and trees tumbling down the slope. Heavy rains are the usual trigger, saturating the ground, softening the soil and adding many tons of weight.
The results of these conditions leave an obvious scar with a story for the observant traveler.