A pocket of old soil is the first thing that comes to mind. It also looks like the middle of the tree is somewhat weak. It could be that thinning the top of the tree where the growth is vigorous will encourage better growth in the areas that are shaded by the upper branches.
The top of the tree looks great, and I see no sign of infestation on the affected foliage.
Other than thinning the vigorous areas, I'd watch the water carefully and continue feeding the tree.