There are massive scientific answers but very simply it's because Autumn is a very revealing time...
All spring and summer photosynthesis is going on in the leaves. Chlorophyll is produced causing the leaves to be green and the leaves are giving off oxygen. This is their season and they are contributing to the good of the world around them.
Then the nights get cool and the weather drier and suddenly what has worked for months works no longer. The flow of nutrients from the tree is cut off and the chlorophyll disappears and the there is no more green color. What is revealed are the colors they were underneath all along, behind the mask of green and/or their colors also reveal what has been stored up or trapped in the leaves when the supply of nutrients ceased.
Trapped sugars produce the reds and leaves that turn immediatly brown reveal trapped poison.
Why do have trouble?
Why do we who love the Lord and seek to serve him find ourselves victims of the same things that plague ungodly, God mocking people?
Why aren't our lives one continued blessing of God after another?
When life is good,days are warm, and breezes are gentle, we are all green.
When life stops working, we show our true colors...
Let your difficulties reveal the sweetness of a confident assurance that you have stored up deep in your heart that God is in control.