This is a common sight in my local beechwoods.
Beech trees have a habit of dropping large branches without much warning. They are not good ones to camp under!
It may be why they earned the nickname ‘widow maker’.
One reason is that the lower branches are shaded by higher ones, leading to their eventual demise.
Another is the significant change of weight in spring affecting beeches more dramatically than others.
The debate goes on though, so if you have any other insights into this phenomenon please get in touch.
(Willows also have a habit of collapsing under their own weight, but the smaller branch size makes them less fearsome.)