Or are they successful because they already have a good team? I think of this as Joe Gibbs leaves after almost 3 years. What makes a good coach or do we overestimate their abilities? I think also of 2 former Washington coaches - Marty Schottenheimer and Norv Turner both with stints at Sand Diego. Who wouldn't like to coach Indianapolis with Peyton Manning? Marty was relatively successful at San Diego but I think that he got lucky with Drew Brees. I also think that Bill Belichek got lucky with Tom Brady (due to injury to Drew Bledsoe - would Brady have been 'discovered' otherwise?).
I think that good coaches don't necessarily have to have a overwhelming winning record -- good coaches serve as good mentors and are able to guide the players to play their best. They are the ones who can put together the "right" players to become a team, rather than the best players. Unfortunately, I don't know enough about football or sports for the matter to know who good coaches really are.
In many ways, head coaches are analogous to fund managers and CEOs. If any of them have a good year how do we know they are really good or that they are just riding a trend or the coat tails of some confluence of luck, good management and a sprinkle of decent coaching assistants?