A. The reason upright limbs of fruit trees with large fruit become horizontal is from the weight of the fruit. The fruit tree which demonstrates this the best is pomegranate.
|This orange tree has not yet produced fruit. Its major branches are vertical.|
The upright shoots of pomegranate bend nearly horizontal after they flower and bear fruit. The weight of the fruit bends the branches downward. To a lesser degree we see this in ornamental trees and nut trees as well as they get older.
In these cases, the weight of the branch as it gets longer and heavier begins to bend the branch into a more horizontal position. Often times young trees are described as being "upright or semi upright" while the mature forms of the tree may be called vase shaped or even round.