Spray it with a soaking rain! I think that's typical with these trees in very hot conditions and sporadic - at best - rainfall. It's nearing mid-August now and most of the trees in my part of the world are dropping leaves due to lack of rain and heat. It's actually normal behavior. Mulberries, sycamores, catalpas - all those with lots of surface on their leaves are particularly prone to have leaves drop at this time of year. It's also more noticeable with them due to the large size of the leaves. Oaks, elms, hackberries, pecans all do it, but it's not so noticeable.
