A key to understanding horse behaviour is to realize how much horses learn by associating one thing with another. This is called classical conditioning. They associate a certain environment with the place something significant happened or typically happens.
Could your horse be linking the cross-ties area with a negative experience? Perhaps he’s sensitive and dislikes vigorous grooming. Maybe he’s had a negative clipping or girth cranking experience. Separation from his pasture buddies is another potential trigger for dancing around in the ties.
On the other hand, could this be the spot where your horse receives his pre- and post-ride treats? Horses paw when they anticipate something. Due to unfortunate timing, if riders dispense the treat in the wrong moment, their horses think the reward was achieved through their pawing behaviour.