Ritzy is almost 2 years old and still occasionally reverts back to the kittenhood "sideways dance" that you described - it is always accompanied by the "RCO" (Random Cat Outburst) once a day romp through the house, up the stairs and back down.... a sign that she is frisky and playful, and healthy trait....

She did this more frequently as a kitten, but still has her moments...I always call this her "happy dance"