No one knows quite why they set off in the first place. But after a 17-month jumbo journey that captured the imagination of people across the world — and continues to baffle experts — a 14-strong group of Chinese elephants is on its way home. What possessed this adventure-seeking herd to turn around, having trekked 300 miles north, and finally head back towards their protected habitat in southwest China ’s Yunnan province? Perhaps they have realised that the grass — and bushes, tree bark, plants, roots and fruit — isn’t always greener on the other side. Or have finally got…