Barry Sanders, arguably the best Detroit Lions football player of all time, stunned the world when he announced his retirement prior to the 1999 season. 

His decision was a stunner, as it came when Sanders appeared to be in the prime of his career. He walked away from the game after compiling over 15,000 rushing yards across 10 NFL seasons. Many wondered what drove the running back to step away from football. 

Now, thanks to a new documentary, Lions fans have more answers regarding Sanders' departure. 

Sanders, now 55, is the subject of Amazon Prime's Bye Bye Barry. In the documentary, which was released Monday evening, he discussed his decision to retire and what drove him to go in that direction.

His decision to retire was puzzling at the time, as he was nearing the NFL's all-time rushing record then held by Walter Payton. With 15,269 career rushing yards, he was just 1,457 yards short of breaking Payton's record at the time of his retirement.

The running back elected to announce his retirement via a fax rather than hold a press conference. He then took a trip to London, which the documentary recreates with him and his sons. 

Sitting in a theater, Sanders revisited the feelings that forced him away from the game. 

"I mean, I think about how much football I had played at that point, how much I had invested myself in the game," Sanders said. "For me, that thing that drove me to continue to play, which was that passion, just wasn't there. There was nothing really left to play for, I didn't see us as any sort of serious Super Bowl contender. I felt able-bodied, sound mind, certainly not according to my dad, but I felt like I had a sound mind and was making a pretty clear decision. I just felt like, in my mind, this is pretty much it."

Sanders stated that he did not consider leaving the Lions and continuing his career with a different team. The Lions' losing history has long been considered to have been a major factor in his decision to retire. 

He elaborated on this during the documentary during a conversation with his sons. Together, they visited London similar to how he did after deciding to retire.

"I couldn't simplify it just to say the losing, because we had been losing for many years," Sanders stated. "But obviously, you do play to win the game. For me to continue to play, I would've had to have had the same drive. I just knew, inside, that was it." 

Elsewhere, the special look at Sanders' life displays his upbringing. The story spans his time in high school through his legendary career at Oklahoma State, where he won the 1988 Heisman Trophy and finally his 10 outstanding years as a Lion.  

There's insight from many Detroit celebrities who admired the running back during his electric career. Among those featured are actor Jeff Daniels, musician Eminem and basketball star Jalen Rose. 

A plethora of Sanders' former teammates, including Herman Moore and Lomas Brown, also feature in sharing their memories with the 2004 Hall of Fame inductee.