I can’t tell from the photo if this is a weeping willow or some other kind of willow, but all willows, especially weeping, are prone to brittle branches and branch breakage because they grow so fast. There is not much that can be done about that kind of branch drop.  Willows also need more soil moisture than many other trees and if there was a period last winter or early this spring with a prolonged dry spell, then that could have caused the branches to die.  I would suggest that you continue to monitor the tree for any further die back and if there is to look for boring insects or some type of scale on the trunk.