http://m.espn.go.com/general/story?stor ... nd=ref~%7B"ref"%3A"http%3A%2F%2Ft.co%2FVgQ5KqXqiX"%7D
Basically he is scared of the long term effects of head trauma and decided it wasn't worth it. Outside of this unfortunate story, between Borland and Willis the 49ers are in massive trouble at LB.