A nasty habit that can form under saddle or on the ground. This action would be completely permissible in the wild where the fight or flight instincts are a matter of survival. Not the case in captivity. Take a look at the history of the horse that has this behavior issue and chances are this behavior is a response to an event or situation that has unearthed the natural instinct. Most, not all, horses with a habit of kicking while on the ground has had some type of encounter where they have felt threatened, confined, or the need to outwardly display dominance. Finding the trigger for this and teaching them with compassion will help remedy the behavior. In the same aspect the horse that kicks under saddle could have several reasons to do so, however as the rider/owner/handler/trainer it is our duty to find the cause. By using a technique where you simplify and control the environment you might find the issue is equipment, placement in a group, or a physical ailment.