According to CBS Sportsline:

Coffee issued the following statement about his retirement: "This has been a tough decision for me, but at this time in my life I feel it's best for me that I move on from football. I appreciate what the 49ers, my teammates and coaches have done for me, and I wish them a blessed season and all the best in the future. 

That's so unfortunate.  Really thought he could be a great NFL back.