When deciduous trees shed their leaves in the autumn, they do it to protect themselves. Water would continue to evaporate through the leaves in the winter sunshine.
Since, however, the water in the ground freezes in winter, the tree's water supply would no longer be guaranteed.
The plant therefore develops phytohormones and sends them to its leaf stalks.
The hormones cause an abscission (separation) layer to form there. It becomes corky and the leaves fall off.