Sesamoiditis is an inflammation of the ligaments that attach and support the two little sesamoid bones on the back of the fetlock.
It comes from abnormal strain in the area. The usual cause is chronic strain from poor/incorrect trimming or shoeing (long toes, under run heels). Therefore the hooves should be looked at first and foremost.
Other causes could be extreme work that causes extremes of fetlock flexion, (race horses, jumpers) or weak suspensory ligaments. (DSLD, or injury) .
Fetlock support boots when in work can help BUT almost always a change in trimming or shoeing helps the most.