Good-Person
Captain Junkie
Personally, I believe that most of them simply do it because they feel that it's something they can actually do because they are the boss and it's their business, so they have the right to do whatever they want to do, to any of their staff and whenever they want to do it without anyone doing anything as a response, otherwise they are going to sack you from the job.
I have myself witnessed many bosses yelling at other people for no reason. However, in many cases, there have been many employees who tried to show an upper hand over their bosses and their bosses started to yell at them in order to show them who really is the boss!