Actually, the term proud cut comes from people colloquailly referring to the epididymis as the "pride" - this is a testosterone producing gland that is wrapped around the testicle and sometimes a supporting ligament. During a standing or prone castration, the vet has to reach up into the body cavity and carefully peel this glandular tissue away and remove it. Just removing the testicles and not the gland, or not taking proper care to peel all the glandular tissue away from the supporting tissue results in "proud cut." Assist in a few castrations and you'll be surprised that there aren't more cases. I think this is way more common than cryptorchidism. Depending on how much tissue is left behind and how much testosterone it produces, your gelding can do a pretty good impression of a stallion.
Most of the OTTBs I've had were cut late and exhibited some stud like behavior, but nothing unmanageable - mostly an unseemly interest in mares and a willingness to court and tease one . I've alway wondered whether it was learned behavior or truly a result of being cut late.
If your horse is managable, I wouldn't worry about it.