A reaction of deciduous trees to stress (lack of water / too much water, too hot / exposed..) is to send a signal to their leaves that they are to be jettisoned. They are removing leaves, that transpire moisture (a cost) so that they can concentrate on their roots (this is simplified).
Prior to their departure from the canopy, these leaves adopt the autumn colours. This is a clear signal that is shown and indicates that an investigation into what is the issue and remedial action could be required.
If the leaves die and are retained on the tree, this is a sign that the stress is so severe that the message to drop has no got through.
Evergreen trees lose their leaves naturally from the inside of their canopy as they are shaded. It is when this is magnified of the loss is from the entire canopy.