What part of the world are you in please? Is it possible, where you are, that some nocturnal creature is eating the leaves at night? Are the trunk and branches of the tree healthy, not showing signs of weeping or soft areas, nothing there that shouldn't be? Inspect the tree thoroughly to check - and the backs of leaves and green stems.

welcome to GoY too.
are the leaves intact? changing colour? as already asked where about in the world are you? have you had a recent drought or very wet spell?

Anywhere in the northern hemisphere, it should be going to sleep for the winter, just now--or, actually, last month or earlier! In the southern hemisphere, have you been having bad windstorms, lately? Has there been any digging or construction going on within the drip line (right under the outermost twigs)?