Your photos show a plant that is doing decently well.  The leaf photo exhibit characteristic senescence (general dying).  My guess is that this plant has not OR poorly established (probably not developed root system).  When there is too much top growth, the tree is "smart enough" to realize that it needs to shed so not to over stress the roots.  In years where the soil is extremely wet, the roots are inhibited hence the plants inability to maintain many leaves.  Sometimes these symptoms that you describe can also be caused by over fertilization with high nitrogen fertilizers.
My suggestion is to ensure good drainage, and use root-stimualting type fertilizers.   DO things to encourage root establishment.