When did you last have some decent rainfall? This could be drought, but the other thing is, did you notice any disease or insect infestation prior to the leaves dropping.

It is hot and very dry here in Essex and your old tree might be finding it heavy going. We laid a patio near an old apple tree. In 1976, a spectacularly hot summer, it died very rapidly. Other causes of leaf browning could be some sort of element deficiency and dryness added. I noticed when we parked near a church hall to go to a club meeting on monday, a huge tree in the churchyard next door was dropping a lot of leaves to relieve stress.