What explains this 13% fall in the Nifty and Sensex in a single day on 23 March?

Actually you should consider lucky that the market did not fall more considering that it had touched 10% lower circuit on opening itself. It was a day when stocks across the board fell, including all the private banks and even the iconic NBFCs like Bajaj Finserv and Bajaj Finance. It was the second instance of trading halt in the Indian market in a span of 10 days. At close, the BSE Sensex plummeted 3,935 points to 25,981 levels; its worse single day fall in history. The BSE Sensex had an advance/decline ratio of 0:30 while the Nifty had an A/D ratio of 0:50. This panic was supported by VIX rising to near the 72 levels. While all sectoral indices were in the red, the private banking index took the worst cut of 17%.