Seems inexplicable at first glance. Didn't have a concussion last year, was an instant star and could have been a superstar this year replacing Patrick Willis and for a long time to come.
The way Borland played in college and then how hard he worked to get his chance in the NFL, he'd be the last one I'd expect to up and quit like that. Reminds me of Ryan Swope's mysterious retirement, hope at least it was purely Borland's decision only with no other factors or influences involved.
If tough guys like Chris Borland are quitting, it's a bad omen for football and the NFL, and I see it helping the Caste System if anything as Whites are more likely to try another sport (as many already have done) or be ordered to quit football by their parents. Plus most are smart enough to have a Plan B in mind if they don't make it in football, whereas blacks generally put all their chips on trying to make it as a pro.