AThere are many possible causes of the behavior you describe. It would take a more detailed history to comment specifically on your horse, but I can make some general comments on this type of behavior.

Physical discomfort–With these behaviors, you’d first want to rule out physical discomfort. Horses that are uncomfortable can often be bothered by other horses coming near. And as strange as it seems, some horses don’t appear to appreciate the stall walls as an effective barrier. From your question, I couldn’t tell whether the stall kicking is toward neighbors in adjacent stalls, or just kicking out and contacting the walls as some horses do when in pain. For a behavior specialist to evaluate the situation, a video (several hours at least) of the horse in that stall under the usual conditions associated with the kicking can be useful