1. A boss they admired was an asshole.
2. They are insecure in their role.
3. They prefer intimidation to leadership.
4. Their life sucks.
5. They lose their way.
6. Promotion chasing.
7. Their management chain is toxic.
8. The Peter Principle.
9. They’re not assholes, they’re just insensitive or oblivious.
10. Madly in love with themselves.
11. They always were assholes.
I've been pretty fortunate to have rarely had an asshole for a manager. I've had a few second-tier managers who were assholes, though, and I think they all fit into this Top 10 nicely.
By far the most common cause I've seen in Promotion chasing. It's the "head in the sand" approach to management. In short, telling the truth is not good for your career. Nobody wants to hear bad news, because their boss doesn't want to hear bad news. When the shit hits the fan, go nuts. Make sure the powers-that-be know you're in total control of the situation, and that you'll make sure heads roll for this awful sandbagging you just took. Management will nod approvingly, and you'll go up a notch in their books.