As in previous versions, I'm sure this is related to the differing skillsets required for the roles in FOF and that both we and the AI know whether a guy will be good at a different role. Here's a good example from the CCFL:
Albert Rasmussen CCFL Staff Page
Rasmussen was a very good OC for multiple good/great offensive teams. Why? Very high scouting, solid playcalling, and solid young talent. That's what you look for in an OC. But as a HC, his motivation is absolutely terrible and his discipline is below average. So, NO ONE is going to hire him to be their HC, because we know he is poorly rated in the key areas there. That league ended in 2053, but it's certain that Rasmussen would have continued on until retirement as an OC.
My guess is the guy(s) you are looking at were strong in motivation and discipline, so they're immediately hired as head coaches. And I assure you that as long as the game works this way, that's what you WANT the AI to do, so the game will be more challenging: hire the absolute best HC candidate as a HC. And the best HC candidate isn't the guy who has been a solid OC for 6 years; the best HC candidate is the guy with the highest ratings in the key categories.