Jonathan Martin became a household name a few years ago when he was a member of the Dolphins and was an alleged victim of bullying from former teammate Richie Incognito. He has since been picked up and dropped by the Niners, before being claimed off waivers by Carolina. Martin has decided to hang it up at the age of 25.
The reason has to do with a lingering back issue, per NFL.com:
After being cut by the 49ers in March, Martin was claimed off waivers by the Panthers. However during the offseason, he suffered a back injury, Rapoport reports. Instead of undergoing surgery and potentially missing the 2015 season, Martin has chosen to retire.