Trees "calculate" which branches to drop based on data they gather as they grow. A competent and experienced arborist knows how to read the tree and simply safely cut off the branches that the tree was already intending to drop so that it doesn't fall on your house or whatever. Inexperienced arborists often ignore this key dendrology fact and may totally butcher your tree. In the process they can create an even larger risk of a tree falling over onto your house during a storm
It's called cladoptosis (basically self-pruning) https://en.wikipedia.org/wiki/Cladoptosis
EDIT: since I'm on the topic of bad tree trimming, NEVER TOP YOUR TREES https://www.arborday.org/tree-resources/dont-top-trees