Unfortunately it may be impossible to know. There are many environmental factors, diseases, and pests that can lead to a slow decline of woody plants. It looks as though this tree has experienced dieback for a while and likely has some internal rot. Often, by the time damage is noticed on woody plants and trees, the damage is already significant and permanent. It's overall structure makes me worry that it could be a hazard or a danger to those below. 

I recommend using the site below to find a local certified arborist, preferably one with a Tree Risk Assessment Qualification. They can help you assess the trees overall health and recommend any options that can help stabilize it or lengthen it's life.
https://www.treesaregood.org/findanarborist/