Shedding of mature leaves from the stem or ripe fruits from the stem is called abscission. Generally a layer of tissue is formed at the base of the organ. For, example, at the base of petiole in mature leaves of deciduous trees. This layer of tissue is called abscission zone. The cells of this zone are thin walled and without deposition of lignin or suberin. At the time of abscission, the middle lamella may dissolve between the cells of two middle layers. It has been shown that abscission zone does not occurs when the concentration of auxin is high.
Because it is then detached from the tree and the gravitational force pushes it down to earth.
because of the impact of gravity, gravity pulls everything down, even at any part of universe you feel gravity, but it is in a less amount,