From Google...
The ligaments are very strong and don't pull away or tear easily. Ankle dislocations often occur along with a break in 1 or more of the ankle bones. In some cases, an ankle dislocation can happen without a break in the ankle bones. In these cases, the ankle dislocation occurs along with a severe ankle sprain.
Ankle dislocation results from severe injury to the ankle. This tears 1 or more of the ankle ligaments. Without these ligaments to hold your bones in place, your ankle bones separate. This can happen in a motor vehicle accident. It may also happen while playing sports, especially ones that include jumping. It is more likely to happen when you have your foot pointed down during impact.