The Pinocchio tree is part of the Lime Avenue, which has been restored by Lord Sainsbury who owns the land.
One explanation of its unusual growth is that the main trunk was originally alone but became hollow over time as the inside rotted. The tree then sent out an adventitious root which took hold inside the fertile rotten trunk and began to grow. Then, as the old trunk broke up and healed around the wound, the lesser trunk became more prominent.
Another special tree has a similar pattern of growth.