With Him (Allah) are the keys of the unseen, the treasures that no one knows except Him. He knows whatever there is on the earth and in the sea. Not a single leaf falls except by His knowledge. There is not a grain in the darkness (or deepest part) of the earth, nor anything fresh or dry (green or withered), but is (written) in a clear record. (Surah Al-An'am, Verse 59).
Most importantly, it's a season of reflection of our finiteness as human beings-
Set forth to them the similitude of the life of this world: It is like the rain which we send down from the skies: the earth's vegetation absorbs it (In Spring), but soon it becomes dry stubble, which the winds do scatter (In Fall): it is (only) Allah who prevails over all things. (Surah Al-Kahf, 45).
Do you not see that Allah sends down rain from the sky, and leads it through springs in the earth? Then He causes to grow, therewith, produce of various colors (in the spring) then it withers; you will see it grow yellow; then He makes it dry up and crumble away (in the fall). Truly, in this, is a Message of remembrance to men of understanding. (Surah Az-Zumar, 21)
Fall is also a time of harvest, a time to share:
It is He (Allah) Who produces gardens, with and without trellises, and dates, and farmland with fruits and vegetables of all kinds, and olives and pomegranates, similar (in kind) and different (in variety). Eat of their fruit in their season, but give the dues that are proper (give a portion to the poor) on the day that the harvest is gathered. But do not waste by excess, for Allah does not love those who waste. (Surah Al-An'am, 141).
Truly these are signs for those who reflect!