Topping, also called "Heading Back" or "Hatracking", refers to the removal of major portions of a tree's crown by cutting branches indiscriminately. Topping severely injures trees. It sometimes kills them outright.
Why would a tree owner ask to have a tree topped? There seems to be two main reasons:
1) Concern for safety - possibility of blowdown. etc.
2) Misguided intent to save money - accepting a low bid in the absence of clear specificatons.
Correctly pruned trees are not often apparent to the untrained eye, whereas "topped" trees are very apparent. The examples of quality tree care frequently go unnoticed while the mutilated victims of topping serve as a standard for tree care.
Topping destroys the natural form of a tree. It stimulates excessive, undesirable growth and quickly defeats the original purpose of pruning. Restorative pruning, over time, can slowly bring back the natural shape after a tree has been topped. A tree's life is sharply reduced by topping and the resultant decay weakens the limbs and trunk.
Topping lowers tree values. A low bid to top a tree becomes the most expensive method when considering the loss of tree value and the increasing maintenance costs required to preserve an unnatural form. Topping cannot make a tree safe, it causes more problems.
Those who ignore proper tree care and top trees, ensure long loss to everyone, especially the tree.