This is a condition known as cladoptosis. Cladoptosis occurs when a tree sheds branches. This is very similar to the shedding of leaves in that in both cases, an abscission layer forms that allows the leaf or branch to drop cleanly. There is no tearing as the leaf or branch breaks free. On branches, the
abscission scar resembles a socket and the attaching part of the branch resembles a ball.
Though sometimes due to shading or injury, in this case, the trigger for branch shedding appears to be a reaction to drought or the alternate wet and dry periods we have had this year. Be
especially vigilant about watering such trees before winter if the soil is dry. (Ward Upham)