Arthritis is something easily seen on an MRI. It is an incurable degenerative condition that can be managed to a degree, but ultimately it wins and the only remedy is a joint replacement (something, of course, that no NFL player can play with).
There no doubt is something to the arthritis theory (which the team and Gurley never directly refuted), but there was more. There were credible reports that Gurley had become less than a good teammate, and perhaps it was that he never really felt very good anymore and it got to him psychologically.
But then the real question is, why would the Falcons pick up a RB with arthritis severe enough to turn him into an ordinary RB? The $5.5 mil isn't huge, but it's a good chunk of cash for a guy who supposedly has arthritis severe enough to hamper his performance and limit his availability It's not like the Falcons have the Ravens' Oline that will bulldoze big openings for him to prance through.
The entire thing is as mysterious as I've seen with an NFL player. But his play this coming year, with a new team, ought to provide some answers.