Anywhere there are "thin soils" (soils in areas that are ledgy or rocky, or heavily stratified by clay, sandy soils etc.) can be dangerous places. Keep off of trees on hillsides in these areas and on windy days, or days just following high wind or ice damaged conditions. These situations will typically loosen up the root ball on many trees and come down soon afterward, especially already weakened trees.
This often causes the condition referred to as "summer drop" where perfectly healthy looking trees just fall over on a beautiful calm summer or fall day.