The primary thing that triggers the changing of colors in leaves is the length of day. However, moisture and temperature play a role as well. For instance, an extreme drought in the summer can delay somewhat the changing of the colors in fall.
Why this is the case is not wholly understood, but it is possibly from the tree not being able to make as much food to store up for the winter, in the case of the drought, so it might be trying to push the envelope a little in terms of making food for a couple more weeks before needing to shed the leaves.
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