Self-Pruning Trees

My son asked why the branches of some trees seem to fall off without any reason.

I found the following quote on the National Association of Forest Industries website:

The self-pruning habit can be a disadvantage in a town, and the choice and positioning of some species need careful planning. The limbs of some eucalypt species are dropped progressively from the tree by a natural process: a fungus weakens the base of the limbs and the growth of the stem forces the branch to break off at that point. The limbs have a habit of dropping without warning on calm, hot days and for this reason many Australians are wary of picnicking or camping under large, old eucalypts. Bushmen refer to these limbs as 'widow makers'.

I also found some research articles on the same topic. John H. Schaffnew says in his paper that some trees develop "brittle zones" and "cleavage planes", which form weak spots in the branches. When the limb grows, it causes these branches to break off. Brittle zones and cleavage planes exist in deciduous plants which allows them to shed their leaves in winter. They also exist in fruits, which is why we can pluck a strawberry, or an orange and not take off half the branch as well.