FII are selling from last 6 months, USD has been showing strength across the board. Panic selling by FII can be justified by strong USD, as most of these foreign players hold portfolios in USD and index they track for nifty stocks is NSE defty index. This index is simple conversion of USD asset to present INR asset, so for foreign players it’s quite common to keep track on USD investments. Recently American economy was registering strong growth, so USD gains were natural but foreign traders who invested in other global markets were losing because local currency we depreciating. Due to strength in USD traders wanted to keep their money in safe heaven assets, ultimately USD demand increased and selling pressure went violent.