Definately something to talk to your doctor about. The ideas about physical damage incurred in youth is something to look into, as hopefully there is a method of recuperating from it. While normally more of an unconscious contraction, perhaps there are ways to strengthen the muscles through synergism of contracting nearby more active muscles. For example, by practising squatting techniques under moderate weight, the gluteal muscles will contract, and they are superfical to the sphinctor.
I think there could be psychological conditions related to many cases of related maladies for this. Subtle body language is often related to emotions after all. For example, someone might associated clenchin or unclenching muscles in that region negatively or positively.
Are you aware of when this happens, a sudden loss of control? If so it could be valuable to review any common occurances preceding it if there is some sort of cause common to all. If it's more of a subtle accumulation you're generally not conscious of, it can be harder to pinpoint. Alterations in posture or mannerism or habit or activity could still affect it maybe?