A foremost example is the creation of cavities in which pathogens thrive. Cavities result from improper pruning i.e. from cutting flush with the trunk when removing branches. Originally trees were meant to grow in groups. A single urban tree without symbiotic relationship with others will develop large lower limbs and co-dominant branches at a young age, leaving weak ones that can easily break off. Another means to enhance the safety is the use of bolt rod, round washers, amon nuts and cables in the correct way.
Legal aspects are e.g. right of way, drive-ways, powerlines, etc.
When calling a professional to work on your trees inquire about credentials, insurance and experience. Remember that wound dressing does not stop rot and may actually increase it. Trees do not heal the way we do. We have organ systems, in trees they are organized in compartments, a defense against the spread of pathogens.