Most commonly unstable ankle starts after an episode of injury, usually an ankle sprain. The main question is whether the ankle ligaments are slack and the ankle therefore mechanically unstable. This can be diagnosed by comparing how stable the injured ankle is compared with the other ankle.
It is possible that the ligaments may be mechanically normal but the ankle nevertheless feels unstable. This is called functional instability because the ankle joint cannot be demonstrated to be unstable by examination.
There are various causes for unstable ankle, either originating from inside or outside of the ankle joint. Within the joint inflamed soft tissue resulting from the injury known as synovitis or an injury to the joint lining may lead to pain / reflex spasm in certain positions of the foot. An injury outside the ankle joint to the peroneal tendons can also be responsible.