Mike Brown is just one of a million example in sports and the workplace in general of someone getting promoted past their competency level. He was a great defensive coach, but he's a terrible head coach because he doesn't know how to manage a game or make appropriate adjustments. As a defensive coach, this wasn't an issue, but it loses games (especially playoff games) for a head coach. Brown is also pretty inept when it comes to running an offense, and thus far has seemed pretty unwilling to bring in a talented offensive coach to run it for him.
Other Cleveland sports examples are Pat Shurmur (who wasn't even a good offensive coordinator and should never have been promoted to head coach) and Romeo Crennel (see: Brown, Mike). Some guys just aren't meant to be head coaches.
For anyone on this board who works for a living, you probably see this kind of thing all the time. Everyone has a competency level, and if they're promoted past it, they're going to flop.