Why do people often shake branches of a tree for getting down the fruits?need scientific reasoning

The stem has just enough strength to support the fruit. By
shaking the branch the mass of the fruit experiences a small acceleration because its
velocity before shaking was zero and then has a non zero velocity during shaking.
Because the mass of the fruit is accelerated this means a force has been applied to it.
During upwards acceleration the stem must exert an upwards force on the fruit as well as
support its weight. If this combined force exceeds the force needed to snap the stem the
fruit falls. This is an example of Newtons 1st Law.