Sometimes, players decide their heart isn't in the game. In 2013, Rashard Mendenhall quit the game abruptly to travel and become a writer. Before that, Tiki Barber quit football in his prime to pursue a broadcast career.
But what's happening among NFL players today feels much more like a trend in motion than a mere coincidence of timing. Since the end of the 2014 regular season, nine players have retired from the game at age 30 or younger. While their reasonings may vary -- and some departures have gone unexplained -- the physical toll of football is a common theme among many of these early retirements.
Detroit's Calvin Johnson is the latest, and most successful, player to walk away with years left to contribute. But he's only the most recognizable name on a list that seems likely to grow.
In Past Year, Nine NFL Players Retired By 30
Patrick Willis, 30
The 49ers linebacker retired after numerous injuries made him unable to compete at the highest level possible. At his retirement speech, Willis also expressed concern about his long-term physical health.
Maurice Jones-Drew, 29
The star running back abruptly announced his retirement on Twitter after last season ended, saying that "All good things must come to an end." He left the game to pursue opportunities in broadcasting.
Jonathan Martin, 26
Better known for bringing to light a culture of bullying within the Miami Dolphins organization, Martin cited mental health and suicidal thoughts in a long Facebook post that announced his retirement. He did, however, mention that the physical risks of playing football created even more incentive to walk away.
John Carlson, 30
After spending his career with the Minnesota Vikings and Arizona Cardinals, Carlson retired abruptly after the 2014 season. He didn't offer a reason for walking away, but Carlson's career was marred by head injuries, including three concussions in the NFL and at least two head injuries in college.
Jason Worilds, 27
The linebacker walked away from football right when he was set to earn at least $15 million in new contract money. The Pittsburgh Post-Gazette reported that Worilds left football to work for the Jehovah's Witnesses.
Jake Locker, 26
Previously the starting quarterback for the Tennessee Titans, Locker quit an injury-plagued NFL career, saying simply, "I no longer have the burning desire necessary to play the game for a living."
Chris Borland, 24
After earning a starting spot in San Francisco's defense, Borland quit after his rookie season, citing concerns about head injuries and the violence of the game.
Calvin Johnson, 30
Johnson has reportedly informed the Lions he will retire from the game, citing football's wear and tear on his body.
Anthony Davis, 25
The 49ers offensive lineman had a weird exit from the NFL, saying he would eventually return but terming his hiatus a "retirement." In a statement, he wrote: "This will be a time for me to allow my Brain and Body a chance to heal."