Cladoptosis is the naturally occurring shedding of branches of a tree. The tree may do this as a reaction to drought, soil compaction or disease. The branch that comes off will have a rounded ball and socket look. Specialized cells form where a break occurs. This is a protective layer that minimizes moisture loss. This will tell you that the tree is self pruning and not a large concern but to reduce the amount of limb drop increase watering and fertilization and reduce soil compaction in root area. Larches, pines, poplars, willows, maples, walnut, ashes, bald cypress, and oaks, shedding of branches is normal, often occurring annually in the fall, similar to the shedding of leaves from deciduous trees.
If the branch does not have this smooth looking break inspect the tree for visible defects and signs of insects. You should seek and arborist as soon as possible as other branches may come down. Different species are susceptible to various diseases and insects so professional advise is key in the correct treatment. Summer branch drop is common in older trees and in species such as oak, elm and ash. Correctly trimming a tree can prevent this hazard. Prune out damaged and low growth branches and those with decay or cavities. On mature trees have the long horizontal branches lightened and shortened and thin out the canopy.