I'm going to need a little bit more information to provide the best answer:
This is a deciduous magnolia, correct?  
When did the leaf drop start occurring?  
How has the tree looked in previous seasons?  (any leaf spots or leaf drop?)
Did you begin watering in response to seeing the foliage problem or do you normally irrigate in the summer?
Any recent activity that would have disrupted the root system (construction, digging, etc.)?
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Hi Brooke, Yes, they are deciduous. The leaf drop started occurring about a month ago. I've only been in my current house since early May of last year, so I don't know what happened in years before that, but last year I noticed leaf drop later in the summer, after it got really hot. I started watering the trees at that point last year, because someone told me they probably needed water. The leaf drop didn't stop, but the trees seemed to recover well over the winter, leafing out fully in the spring and the leaves looked healthy to me. I didn't do any watering from fall through early summer. But then the leaf drop started again, before it even got hot this year. Regarding activity that might have disrupted the root system: Before I bought the house, in April of last year, the sewer pipe from my house to the street was replaced, so that might have disrupted the roots, but the digging wasn't that close to the tree most affected by leaf drop. Thanks for any help you can provide!

So, Brooke, any more insight into what's going on with my magnolias?

I wouldn't recommend spraying a fungicide at this point because the symptoms on the leaves in the photo don't suggest a major leaf disease problem.   The leaf drop is most likely is the tree's response to the recent hot & dry weather (Magnolias generally have a more shallow root system so are more affected by heat and water stress).  The region also experienced a drier spring than normal, so supplemental irrigation may need to be applied before you start seeing symptoms next year.  Hard to know for sure, not being familiar with the growing environment, but if the root system has been constricted, damaged or if the soil is compacted in the root zone then the tree will be less likely to with stand heat and drought conditions.  

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Yes, that helps. Thanks very much! - Mary