From Goldman:
Metals prices have declined by 12%-17% since late October. Over this period, China’s economic data for October has disappointed, the US dollar has strengthened on a trade weighted basis, and the broader commodity complex has moved lower, including most notably, energy prices.
What has also occurred since late October has been an eye catching rise in Shanghai Futures Exchange open interest across the metals complex – for copper, it has been the largest increase in Shanghai open interest in 12 months – since the 1Q15 collapse in Chinese metals demand.