Leaves that have been invaded by diseases or insects can drop for lack of light or from mechanical damage. This often happens in Montana toward the end of summer. Because the season is near a close anyway, you don't have to spray for the pests. Defoliation at this time does little harm. However, if the defoliation is severe and recurs every year, you should consider treatment with appropriate fungicides.
Broadleaved trees can tolerate a few years of defoliation with no long term damage.
Evergreens defoliated for only one year usually die. Repetitive defoliations every year because of root rot, a girdling root, or other root damage can weaken the tree over time, finally killing it. Poor soil conditions, such as backfilling, compacted soil, or waterlogged soils, also weaken the tree. In these cases, leaf drop is gradual, not sudden. Discolored leaves, whether they fall or not, are often a sign of chemical damage.