What's with the Sycamores?

Every years the leaves of our street Sycamores (actually London Plane Trees) start drying up and falling in August--I have to keep raking them for weeks. This year it's mid-October and they're still mostly green, even though we had a dry summer. Is this happening all over or just to our trees? Is it a diminished response to some disease or due to some environmental change. Do the trees know something that we don't? An inquiring mind wants to know. Richard K.