Often this is a sign of instability. There are a variety of reasons you may be experiencing instability in your knee including strain/sprain, soft tissue tear, muscle imbalance, a nerve condition etc. In the case of osteoarthritis, as the space between the knee joint becomes smaller from the breakdown of cartilage and loss of fluid, the ligaments that are some of the main stabilizers of the knee joint do not actually shorten. This allows the joint to move more than it should.