Yes, when you stop wearing your braces, the bone is still in the process of remodelling. Unfortunately, teeth always follow the path of least resistance, so if nothing is holding them in place, your teeth are liable to move.
Everyone needs a retainer immediately after a clear aligner treatment period. Whether you've had extensive orthodontic treatment to fix crowded and misaligned teeth or if you've had shorter partial treatment, a retainer will keep your teeth from reverting to their original position.
Know that wearing a retainer is much easier than wearing braces and allows you to retain your perfect teeth and smile.