Osteoarthritis of the ankle can occur as a result of the natural aging process and biologic changes that cause joint changes. This is called primary osteoarthritis (OA).
More often in the case of ankle OA, there has been an injury or previous trauma of some kind. This could be an ankle sprain, joint infection, or fracture. Do you recall anything like that in your past? It could have happened years ago.
Anything that can disrupt the stability of the ankle can result in OA. Over time the structures bear the load of your weight unevenly long enough that the cartilage wears away or even tears. You can end up with OA. OA of this kind is called secondary OA.
Most people with stage 2 or 3 OA of the ankle once played sports either competitively or for recreation. Ankle sprains during active play are the most common event linked with OA years later.