I recently watched a german documentary "Das geheime Leben der Bäume" [1] where they visit this exact tree. Highly recommend it. The documentary is based on a novel of the same name by the author Peter Wohlleben [2].

[1]: https://www.3sat.de/film/dokumentarfilm/das-geheime-leben-de...

[2]: https://www.amazon.de/Hidden-Life-Trees-Communicate-Discover...

Mostly, it is due to weather and the direction of sunlight (and due to pests, etc.). Also leaves at the top, those on the outside get more exposed to sunlight and the plant allocates these leaves more chlorophyll for synthesis (hence green). Comparatively, leaves on the inside do not get much sunlight and serve no purpose, so to say. So the plant/ tree sucks the chlorophyll back from these leaves. Now such leaves which do not take part in synthesis can expose the plant to too cold/ too hot weather due to their surfaces area. So the plant/tree sheds them. Of course too severe weather will impact all leaves so in winter etc trees are bare.

If interested, I highly recommend (not a pure science book) Hidden Life Of Trees https://www.amazon.com/Hidden-Life-Trees-Communicate_Discove...

I'd recommend this book, it explains a few interactions between species of animals / insects / plants and shows how everything is more of less connected in ways we can't imagine

https://www.amazon.com/Hidden-Life-Trees-Communicate_Discove...

I just recently read a great book called The Hidden Life of Trees by Peter Wohlleben [1] that touches on the subject of a social network among trees via a fungal network known as the "Wood Wide Web" [2].

[1] https://www.amazon.com/Hidden-Life-Trees-Communicate_Discove...

[2] https://www.nytimes.com/2016/01/30/world/europe/german-fores...