When these trees get to be around 600 years of age their massive crowns start to die back because the root and vascular system is no longer able to maintain the large crown.
This allows light to reach the lower inner crown, which in turn stimulates dormant buds to grow, eventually creating a new smaller lower crown.
The old branches which formed the original high crown die off, but on species such as oak and sweet chestnut, both of which have very durable wood, these dead branches remain for decades, even centuries. This is what gives the tree its stag headed appearance.