Tree topping removes large portions of a crown by cutting major branches or stems back to stubs or unsuitable lateral branches. It can create long-term structural and health problems rather than solving them.
Large wounds
Topping creates large exposed cuts that a tree may have difficulty compartmentalizing, increasing opportunities for decay.
Weak regrowth
The tree can respond with vigorous shoots near the cuts. These shoots may become large quickly but can be more weakly attached than the original structure.
Better alternatives
Pruning should have a defined objective such as deadwood removal, clearance or selective reduction. Where a tree is too large for its location, removal and replacement may sometimes be preferable to repeated topping.