Staying steady is not one skill. It’s three sources of information agreeing with each other: your inner ear, your eyes, and the sensation coming up from your feet, ankles and hips. Your muscles then have to react fast enough to act on what they report. When one of those sources goes quiet, the others quietly cover for it. When two do, walking starts to take real attention, and attention is a limited thing.
Which means the reason is different for different people. It can be the inner ear. It can be nerves in the feet that no longer report position clearly. It can be legs that lost strength through a slower stretch of life. It can be a neurological condition like Parkinson’s or a stroke changing how the signal gets through. It can be a medication with a side effect nobody flagged. A cause-first evaluation is how we find out which of those is costing you the most, because the training that helps is different in each case.