The selling started because of sell-off in the other Asian markets - especially the sell off in the Hang Seng (Hong Kong).
Amongst the Asian markets,
only India and Hong Kong have shown extremely sharp up-moves in the last few days. Thus,
both are also expected to correct in a similar manner.
Today, the Hang Seng index closed nearly 5% down from the day's top and 2.5% down from yesterday's close.
India recovered as buying again emerged at the end.
This rally is completely driven by capital inflows from overseas.
The high volumes are scary and may be a sign of a blow-off top.
http://www.investopedia.com/terms/b/blowofftop.asp
Short-sellers are continuously getting trapped and taking the markets even further up. Shorting against such strong momentum is suicidal.
Such momentum is unsustainable. I feel the markets may top-out very soon, but it is very difficult to predict when the top will come.