It is a crime against nature. Topping is a worldwide tree mutilation practice. The main reason is that people do not understand how trees function. Some effected parts of the system are the non-woody roots, the woody roots, and many microorganisms within the soil. The food for the woody and non-woody roots come from the top.
Many microorganisms also get their food from the tree.
Some demons of D, which begin with the letter D, are depletion, dysfunction, disruption, decomposition, die and death. Also topping affects the stem because this often leads to top rot and long cracks. Often lightening gets blamed. What we have seen is wounds from topping weaken tissues from the trunk to the roots. Then pathogens infect the weakened wood.