I found Scott's post to be quite thoughtful. He actually lists 11 reasons. My favorite is this one, although all are on target. The mental health of managers is something that people don't think about enough (and it is especially tough to be a boss these days, and perhaps those of who encounter an asshole boss who is usually civilized should cut them slack):
Their life sucks. What percentage of people are miserable in the corporate world? I think 20-30% is a safe bet. If you’re miserable, you tend to inflict your misery on those who have less power than you do. If your life sucks badly enough you won’t even notice how rude you are to waiters, assistants, and sub-ordinates. It may be nothing personal, or even work related, these people simply have a volcano of negative emotions that must escape somewhere, often in eruptions that they can not control. Just be glad you’re not their spouse or offspring.
In addition, as I talk about in The No Asshole Rule another reason that managers become assholes is that -- as a growing pile of studies shows, see here and here -- just giving people a little power can make them more focused on their own needs and wants, and less focused on the needs, wants, and actions of others.
I would like to see some research on this, but I am hypothesizing that more employees have asshole managers than just a year ago or so. If you think about the economy, people are stuck with an asshole boss have a much harder time leaving for another job. Those who are stuck are more afraid to fight back against an asshole boss because they are afraid that they will move to the head of the layoff or "performance based" firing list (See this Dilbert cartoon -- people who complain or fight back are among those who make bosses 10 squirm), and finally, on the life sucks theme, if you as a boss are getting your budgets cuts, being ignored and abused by your own boss, facing the stress of laying-off people, and so on, these are all things that drive even the most upbeat manager into a nasty mood. So I fear that the asshole are on the rise and the power to fight back is plummeting in too many workplaces. In fact, I would be very curious to hear from those of you out there in real jobs of examples that refute or support this hunch of mine.
Finally, I have a little reminder for both asshole bosses and their victims. For victims, this too shall pass, it may be unwise to fight back now, but it is a good time to patiently gather the facts and weave together a coalition of fellow victims and supporters. Bide your time and protect yourself as much as you can -- the day may come when things change, when you have more options, or your boss losses power, and you can fight back. As for you asshole bosses out there: You may believe that your nasty style is helping you maintain control via intimidation, and perhaps it is (for now), but your enemies may by laying in wait -- and you may reap what you are sowing right now.