The following is information on leaf drop in trees in midsummer. This is a common occurance during dry periods in many species. However, excessive premature leaf drop can be a cause for alarm indicating too much plant stress. the following is some additional information.
MID-SEASON LEAF DROP
When the leaves of large shade trees drop during mid-season, it typically causes alarm to concerned homeowners/clients. With the ground littered with spent foliage, the conclusion often is that “their favorite shade tree is dying!” Linden, birch, and sycamore trees are often most susceptible to mid-season leaf drop. In a majority of cases, this is a normal physiological growth habit for these species. The trees commonly drop foliage in mid-season in order to reduce leaf surface area and subsequent water loss. This leaf shedding ability is especially important during typical summer droughts or when water availability in soils is limited. Neither tree health nor tree growth is usually affected.
With this said, excessive premature leaf drop can be a sign that trees are undergoing too much stress. Plants with compromised root systems will be the most affected. If you suspect such a problem, examine the root system and take measures to reduce stress. This might include use of a tree spade to aerate compacted areas, additional watering where rooting is poor, or site corrections such as improving drainage (poor drainage can affect roots in wet periods leading to additional stress when soils dry out).
Some information from the Plant and Pest Advisory newsletter from Rutgers University, other from Gordon Johnson, Extension Horticulture Agent, UD.