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.