The tree leaves are attached to the tree through small stems. There is a force of cohesion (attraction between molecules of the same kind) with in the leaves and a force of adhesion between molecules of leaves and the stem.
There is a force of gravity on the stem and the leaves. This is small compared to cohesive and adhesive forces.
When the leaves are green, the cohesive and adhesive forces are high. When the leaves are dry or dead, the cohesive and adhesive forces are very less. The leaves turn brittle.
When the tree is shaken vigorously, there is a tangential force or shear force perpendicular to the length of stem of leaves. This shear force tries to break the connection between the leaves and the stem. When the shear force is more than the breaking limit, then the leaves get detached from the tree.