When people report repeated ankle sprains, the condition is referred to as functional instability. You can walk on that foot, but as you described, every now and then without warning, the foot and ankle give way. This condition is the most common long-term problem after an ankle sprain injury.
Scientists aren’t exactly sure what causes this to happen. The obvious answer is that supporting ligaments damaged in the first injury aren’t holding the ankle in place. But there must be more to it than that, or the ankle would give way with every step you took.
There are a couple of theories to explain FI. The first is known as articular deafferentation. According to this theory, tiny receptors in the joint capsule and ligaments around the ankle are damaged. These are called mechanoreceptors.
Ankle stability depends on the muscles around the ankle to react quickly to sudden movements. Enough muscle tension is created to prevent the ankle from going too far in one direction of the other. The mechanoreceptors signal the muscles when the joint is in danger. The muscles respond to stabilize the joint. Damaged mechanoreceptors don’t allow a fast enough response and down you go!
Another idea is the feed-forward motor control theory. Recent research has shown that the body may react to certain positions and movements ahead of time based on past experience. So for example, when you go to take a step, the muscles contract to position and hold the ankle in just the right spot. The goal is to stabilize it and prevent injury.
After injury, the muscles may anticipate injury and start to contract too early. The intent is to protect the joint but the result is abnormal motor control and giving way of the joint. Sometimes this problem can be corrected with a fairly simple program of exercises and activities. A physical therapist can help you with this.