leaves fall from trees for as usual rules of nature.

The act of the leaves falling from the trees is called abscission. A small layer of scar tissue (of sorts) forms and the leaf is starved of nutrition, eventually dies, and falls.  This happens to deciduous trees because throughout the winter, the tree becomes dormant and must maintain all the nutrition that it can.  Leaves, in turn require additional nutrition.  The leaves falling is the trees way of staying alive during the long winter months.  In addition to this, in the event that a winter storm (snow/wind/ice) arrives and the leaves are still on the trees, the wintry mix would weigh on the leaves, enabling the breaking of the trees branches.