What formed this rock slide formation near Hyndman Peak Idaho (photos)?
It is a normal rock slide, AKA a landslide, they happen all the time.

If a slope is not stable the rock collapses, landslides are very common and can be triggered by almost anything from rain, to imperceptible quakes, to simple erosion.
It could be rotational or translational the two blur together anyway. Most likely just a shallow rotational landslide.
The lower areas of a mountain being made of debris is extremely common, it is called a Scree slope. Where do you think all the rock that is eroded off the upper portions go?