Ask Mr. Smarty Plants

In fall, as the leaves change color and begin to fall to the ground, the process of decomposition begins.  There are many micro-organisms that participate in this process and molds make up a large part of them. Before a plant sheds its leaves it withdraws nutrients, carbohydrates and water from them, closes off circulation from the plant to them and they fall to the ground and finish drying out.

When there is a hard frost, leaves that have not been through this process suffer from damage as the water in them freezes and bursts the cell walls in the leaves.  That is why very delicate plants like impatiens turn to mush.  Broad leaved evergreens do not suffer the same fate as they have stonger cell walls and have adapted so that as temperatures fall, the concentration of sugars in their sap is increased.  This prevents the formation of the ice crystals that burst the cell walls.

What is likely happening is that you notice the smell when the temperatures rise after a hard frost and the leaves that have frozen are starting to decompose.

Ahh, the smells of autumn: wood smoke, apple cider and rotting leaves!