The Yen is trading +1.6% vs the US dollar today. The US Dollar / Yen spot rate took out important support at 110 yesterday and is down big today. Remember that this index shows how many Yen one Dollar can buy...so as the yen becomes stronger one dollar can buy less of them so the index falls.
Here is a shorter-term picture. In the world of currencies a 1.6% move is a very big one.