During the hot, dry conditions of summer, numerous trees will shed some of their leaves. A good example is tuliptree (Liriodendron tulipifera). I (think) I remember flying in from the Pacific Northwest in August one dry year and as we got close to landing, was able to pick out the tuliptrees due to their earlier than fall color yellow leaves interspersed on the tree among the more prevalent green.
Today, I was walking in Wooster in northeast Ohio, and the ground was littered with fallen leaves of tuliptree. It even seemed like some of them were sweating in the 90+ temperatures! Not really, just honeydew excreted from sucking insects.
I checked the OSU Ohio Agricultural Research and Development Center in Wooster weather records, and from May 1 until now the average rainfall for this period here is almost 13 inches. Actual this year: 5.11 inches. So, no surprise about the yellowing and falling leaves. Stressful to be sure, but probably of no great import to mature trees. For newly planted trees, though – watering during dry periods is essential if rainfall does not provide the inch of weekly moisture trees need.