When your horse is standing, are his hind legs very close together? The muscles on the inside of the hind legs may feel stringy or tight. Horses that do sliding stops, whether under saddle, or in free play, can strain the psoas or semi membranosous. On a touchier subject, this strain can appear when a horse carries a rider that is too heavy for him.
Another cause of interfering can be connected with the gelding scar. See post from February 25 for more information.