Step 1: Good, smart white NCAA player goes to NFL for maybe 1 or 2 years if he is lucky, while his black counterparts go become all-pros and make millions for 10-15 years.
Step 2: White player inevitably gets cut, if he gets a camp invite, and goes back to his college as a grad assistant.
Step 3: White player's skills and football savvy make him climb the coaching ranks, and gain experience while black players of his age are using the 70-30 black-white ratio in the NFL to their advantage, and gaining playing experience.
Step 4: After lengthy career, black player decides to retire from the sport. He still wants an income source related to football, so he has a few options - be a coach, be a "color" commentator for ESPN, NFL network, or one of the broadcast networks that cover pro or college football, or sign autographs at sports memorabilia conventions.
Step 5: So the black former player decides college football coach. The best he can hope for right off the bat is a position coach at a major university. He still is at a huge disadvantage to his white counterpart, who is now a 10-15 year veteran coach, who is in his mid to late 30s. He may even have a head coaching job at a major university by now, and who knows - could be the one hiring the ex-pro-bowler to his position coach slot.
Step 6: The NFL hotshot tries coaching for about 5-10 years, noticing a bunch of guys are still far more experienced and skilled coaches, and 15 years his junior. The white head coach is now making $1-4 million/year at his new top-level head coaching job, and the NFL player decides he should take up golf or buy a ranch somewhere. After all, his bank account is bursting at the seams, and he didn't realize how much work this coaching stuff would be. He had a fun couple years coaching, but now it's on to something else.
OK, this is just one example, but I'm sure it's happened a couple times. I think it's safe to say that the caste system for players contributes quite a bit to the inverse system for coaches. Not only do some players become coaches after college, but some do it after high school, after the local colleges don't offer them a scholarship. They then join the high school coaching staff, and move up the high school ranks. Many then go on to college ranks to be some of the most legendary college football head coaches, and some even go on the NFL.