Question
I planted a Magnolia in our front yard 15 years ago.  This year and now, in particular, the tree is dropping an enormous amount of leaves.  We have an irrigation system, so I don't think it's too dry.  It does have an Oak in front of it, shadowing the light quite a bit.
Maybe you can help.  Thank you.

Answer
A magnolia is an evergreen or non-deciduous tree. All plants and I mean all plants drop leaves sometime. What is probably happening with your tree is that magnolias drop leaves in the spring early summer. The reason being is that as the tree grows the interior leaves on the tree or limb get shaded out. Once this happens it does not live (the leaf) because of the lack of light which is essentail,dies. Look out in your yard and you will see very tall trees with no lower limbs. Why? As the tree grows the lower limbs can not keep up. As less and less sunlight makes contact the limb eventually dies and drops off. What you are experiencing with your magmolia trees is normal. However, there will be seasons where the trees grow more or less due to enviromental factors(rain, temperature,etc.) which may lead to more leaf droppage or less.
  
The sprinkler may not be enough water for the tree. I would water with 1 inch of water per week-place a pan under the tree and turn the sprinkler on and when the pan has 1 inch in it stop. This is called deep watering and will help the leaf drop.