One problem that's quite common is that new managers are given very little training or mentoring - the company just promotes a senior developer to management and expects them to somehow figure out how to do it well (at least that's what happened to me). However, management requires a very different set of skills than programming, and, like programming, you can't just learn these skills in a few weeks (especially while working around the clock to try to meet a product deadline that's now your responsibility).

Unfortunately, the mistakes a novice manager can make - hiring the wrong people, irritating good developers to the point where they quit the company, etc. - are much harder and more costly to clean up than the kinds of mistakes you can make as a developer. For one thing, there's a multiplier effect: your actions suddenly affect more people.