NFL fans were shocked last year when former San Francisco 49ers linebacker Chris Borland decided to retire after his rookie season, calling the NFL "too violent and destructive." While not everyone shares his viewpoint, it's clear that the physical toll this game takes is starting to catch up with its players.

Detroit Lions star wide receiver Calvin Johnson, just 30-years-old, headlines a crop of young-ish and talented NFL players to hang it up this offseason. Johnson said he was "at peace with" his decision, but his choice is representative of a growing trend in the NFL.

Last month, former Seattle Seahawks bell cow running back Marshawn Lynch, 29, announced his retirement during the Super Bowl and New England Patriots linebacker Jerod Mayo, 30, followed suit. While neither player's situation can be closely compared to Borland's, it's clear that more and more NFL players would rather walk away from the game a few years earlier rather than wait for the wheels to fall off.

Lynch likely would have generated a solid amount of interest in free agency this year despite missing nine games due to injury this past season. Before that, Lynch had produced at least 1,200 rushing yards and double digit touchdowns in four straight campaigns. He may not have been the same Pro Bowl caliber player he once was, but he could have been productive in the right situation. For him, though, the right situation was no longer on an NFL field.

Mayo's situation is a bit different as he has finished the last three seasons on injured reserve. But the former first-round pick and two-time Pro Bowler has the pedigree that contenders are looking for in free agency. Had he wanted, he could have given it another shot and found himself on some team's roster come training camp.

But neither player felt the need to continue fighting. Johnson left $16 million on the table because of the nagging injuries that had piled up over the last few years. This is a guy who caught 88 balls for 1,214 yards and nine touchdowns this past year. We're not talking about an over-the-hill Terrell Owens trying to make one last comeback.

The dangers of concussions and CTE in football has been well documented. We are aware that NFL players are at a higher risk for long-term brain damage than the average Joe. We have no idea if that thinking played any role in the decisions of this year's retirees, but we know this reality is out there.

"I mean, if it could potentially kill you - I know that's a drastic way to put it, but it is a possibility - that really puts it in perspective to me," Borland said last year. "To me, it just wasn't what I wanted to do."

For the second consecutive season, good players with good football left in them have walked away from the game. For now, it's the exception. But sooner or later, it could become the norm.