Probaby lack of rain. The trees will drop their leaves early.

I dunno. Many of my cucumbers' leaves are suddenly yellow and some of the dahlias' leaves look got toasted (straight to brown & shriveled). We've been pretty lucky the last couple of weeks with thunderstorms. Perhaps they had just gotten used to drought and suddenly: moisture :) I'm pretty sure it's not fall being around the corner. Tulip poplars might turn yellow this early, but not perennials & annuals.