If you are wondering why Rex Ryan and Jack Del Rio were among the first coaches hired this off-season, and why John Fox might be next, then you have not been paying attention to recent NFL history.

It used to be that recycled coaches had no chance of success and, in fact, in the first 31 years of the Super Bowl era only two coaches — Don Shula and Weeb Ewbank — won the championship with a team other than their first.

In the last 17 years, however, that has happened 11 times, and it’s no accident.

The changing nature of the game — primarily free agency — has skewed the equation. With rosters turning over faster than they ever have, teams frequently need a leader who has been through the grinder himself, learned something, changed jobs just like many of the players, then started over with that experience.

This is not to say that Ryan or Del Rio will be the next Bill Belichick, but raise your hand if you thought Belichick would someday be regarded as one of the greatest coaches in history when Robert Kraft hired him for the Patriots. Kraft acknowledged that wise voices in the league office even asked “Why?” when he picked Belichick to coach the Patriots and tried to talk him out of it.

Belichick’s first head coaching gig, in Cleveland, was so toxic that some blamed him for the Browns moving to Baltimore. Look at him now.

Or look at Pete Carroll, who was fired twice from previous gigs, including once by the Patriots, before achieving success in college and now with the Seahawks. The New York Jets once let him go to be replaced by a buffoon named Rich Kotite, who proceeded to go 4-28 in two years before he was fired, too.

The conventional thinking used to be that a coach was most likely to be successful in his first job because (1) owners used to have more patience, and (2) it was harder for a recycled coach to put his desired staff together because the assistants he started with and probably still wanted had dispersed to other jobs.

But the game over the last couple of decades has gotten more complex and more specialized. Free agency and the salary cap have changed team dynamics, and mounting evidence shows that it has become tougher on new coaches who must learn as they go.

“A good part of (succeeding) is experience and understanding the so-called dynamics between ownership, management and coaching and the media, and I think that exposure and experience (to the NFL) certainly serves somebody very well,” Bill Walsh, the late Hall of Fame coach, once told me.

Notice Walsh used the word “experience” twice.

No one, of course, has done a better second act than Belichick.

Kraft knew Belichick from his days as an assistant coach with the Patriots and ignored the entreaties from league higher-ups to stay away from him. Clearly, Belichick learned from his first experience when he tried to adapt the personality of his mentor, Bill Parcells, and was intractable in his dealings with players, media and others in the organization.

With the Patriots, Belichick has shown a more human side, and he’s a better coach, too.

Carroll was hardly a failure in his first two coaching jobs, with the Jets and the Patriots, reaching the playoffs twice in four years and finishing with a plus-.500 record. Some owners thought him too rah-rah and better suited for college, but after achieving success on that level, he seems to have done pretty well in Seattle.

Mike Shanahan and Tom Coughlin won two Super Bowls apiece in their second jobs, although Shanahan likes to say his first gig, with the Raiders, didn’t count because Al Davis didn’t really let him act like the coach. Dick Vermeil, Tony Dungy and Jon Gruden also were second-time-around Super Bowl champions.

Now, Carroll and Belichick are favored to meet in Super Bowl 49, which would mark the second year in a row that two previously fired coaches faced off in the season’s final game. Such a meeting would assure that, for the 12th time in 18 years, the NFL’s championship coach was a guy who had been fired from a previous job.

Sure seems like a trend.

Ira Miller is an award-winning sportswriter who has covered the National Football League for more than three decades and is a member of the Pro Football Hall of Fame Selection Committee. He is a national columnist for The Sports Xchange.