One of the main reasons people leave their companies is their manager. They can’t stand working for a boss who is cruel, rude or small-minded—in other words, a jerk.

These managers likely earned their positions by being good employees, but somewhere along the way they went bad. What happened? Tomas Kucera, VP of Business Operations at SolarWinds, answered the question in a recent posting on his blog, “The Geeky Leader.”

A bad leadership example.They may be emulating the behavior of their own boss. Rewarding bad behavior with promotions reinforces it, leading to a toxic company culture.

Insecurity. They may feel they don’t have the skills to do the job and end up going overboard trying to appear like a strong leader. Combat this tendency by promoting people to management because they are ready, not because they are great at doing their current job.

Low emotional intelligence. They may be highly technical people who are experts in their fields but never learned to communicate effectively or understand people. Provide training in how to communicate, manage and lead before you ask people to do so.

Stretched too thin. Some managers get promoted but still keep doing their individual contributor job. This leads to competing priorities, as they focus on their old job, which they are good at, instead of learning to be a good manager.

For more on why managers become jerks and ways to mitigate the problem, see Kucera’s full blog posting: “Why Good Employees Become Bad Managers.”