Here is an interesting table listing the current head coaches in the NFL and the first position groups they coached in the league. Todd Bowles is one of eight head coaches who got his start coaching in the secondary (although his first job was as Jets secondary coach in 2000, not Arizona's in 2008).
The eight head coaches who started as secondary coaches is the most for any position. Quarterbacks coaches came in second with seven. This makes me think about baseball managers. There is a popular theory that catches make the best managers in baseball because they see the entire field from their position. I am not sure whether it actually works this way in practice, but a lot of people believe it.
Thinking about football, the same is true of quarterbacks and guys who play in the secondary. The quarterback runs the offense. He has to know what is happening everywhere on the field. Meanwhile, the secondary is the last line of defense. Many players back there also have to be able to scan the entire field.
I wonder whether that has to do with so many secondary and quarterbacks coaches eventually growing into head jobs.