Trees fall because their roots cannot hold them upright. If the roots are starved for space, water, air, and nutrients, you will see signs of weakness above ground (like the growth of Resurrection Fern on the main branches shown in the image). If the main feeder roots of a tree are damaged by construction, a tree will fall. If the tree, with its enormous weight and mass, is leaning at an angle which makes it impossible for the roots to hold their ground, the tree will eventually fall. If a tree is badly diseased or infested above ground, the roots will be affected also. If a vehicle drives over the soil above tree roots a couple of times, a tree will eventually fall. Healthy roots are the key to safe trees.