Question
Is there a good reason why one of my cottonwoods has leaves that are turning yellow and falling this early?

Answer
More than likely the weather. Cottonwoods are one of the first trees that will drop their leaves in the Fall. If the weather is hot and dry they seem to compensate by dropping leaves. It could also have a leaf disease (leaf rust or spots) that will cause the early leaf drop. If this is the case fungi that infect leaves late in the season do not really do much damage to the health of the tree. I would not worry about this early leaf drop of cottonwood trees. .