Many causes. They range from stretched out or torn ankle ligaments (usually laterally) to nerve problems causing weakness of the muscle that prevent your ankle from twisting the tendon ruptures/problems. In addition bone structure problems such as tarsal coalitions or varus deformities of the hind foot. Sometimes just unrehabilitated muscle strength with loss of proprioception. Get evaluated to sort it out.
You have. Chronic lateral ankle instability. You can wear a brace to help prevent further sprains or have it corrected surgically. Therapy may help too.