Why Leaves change color:
When the air temperature drops the chlorophyll inside the leaves starts to break down and this reveals the other pigments that also exist naturally within the plant. Some example pigments includes carotenoids and xanthophylls which usually give a yellow and or orange color to the leaves.
...They all fall down:
At the same time as the chlorophyll is breaking down, a layer between the leaf and the main stem called the abiscission layer begins growing, which cuts off the water supply to the leaf. This eventually causes the leaf to fall off but before that happens, the sugars within the leaf also break down into anthocyanin which can give leaves that dark red color.
This seemed seasonally relevant,