I've been experiencing this problem also. The injury theory may hold some water because about 3 years ago I fell from a ladder and broke my foot. This may coincide with when my penis started retracting but I didn't really start noticing until a year ago and it doesn't happen all the time. I am 53 yrs old. I haven't exercised much since then, though I used to,I'm probably about 20 lbs overweight, but I've always been on the heavier side. Point being I don't think it's exercise or weight that causes the problem. I think it may be more attributed to stress on my body from the injury. From earlier posts I see that for some it's been, or is, happening since youth, but even some of those posts mention physical stresses put on the body so maybe there is a real correlation there. Maybe a Sports Therapist could shed some light on this problem.