A. Sometimes they can get so dense that internal shading of the canopy will cause leaves to die and drop. These are fairly messy trees due to leaf drop and dropping of their berries. If the tree in the drier landscape is more open and the tree in the back is more dense then this can be the reason.
Other reasons might include pest problems such as aphids which can cause the leaves to be sticky or shiny and leaf drop if there are in large numbers. If the leaves are sticky, try spraying with repeat applications of soap and water or insecticidal soaps. Aphids should disappear during hot weather but reappear again during cool weather. You may also see a lot of ant activity that are working off of the aphids sugary honeydew droppings.
You can try to open up the canopy with some branch removal (thinning of the canopy by selective limb removal) and admit more light inside the canopy. This however will probably cause some suckering on the limbs if too many limbs are removed.
Another possibility could be overwatering. If the tree is watered too often it is possible to develop root rots and lead drop will occur and usually branch dieback.