I think it's the very wet late season and accompanying warm soil temps and the lack of cool nights to start the trees producing the sugars that create the vivid colors. I think it's a similar process to how Maples are better sap producers in the spring when it's warm days and cold nights. Lets see if the trees that are still summer green turn rapidly now, especially after the next two nights, and if they get vivid or just fade away. I'd say that it's well over half of the trees in the suburban neighborhoods by me are still summer/late summer green while ~35% turned and have dropped/are dropping their leaves.