Basket selling by some very large institutional seller executed at bid rates by institutional broker like Enam, IDFC capital etc.
No one in this world know who the seller was, who the broker was who executed the trade. Experienced traders know one thing that some large guy is swallowing all the bid and selling hugely, so exit all long trades.
When an institutional seller sells, the seller swallows all the bid rate in the order book. Within 5 minutes, Nifty Future felt from 6070 to 5976 as all the bid rates in the order book were swallowed.
Everything else is a myth. Business channels will give you a reason because it was expiry day this selling happened. On expiry day, Nifty could move up as well as it has done in past series.
Technical traders will give you all kinds of reason- 61 Fib got hit or SMA crossover or RSI hit magical 80 ! These are man made human reasons.
No one in this world knows when an institutional seller will sell or buy. Only the brokerage guys in institutional team knows it when they receive the order which is few second before the selling starts but they dont conduct their own trades to avoid conflict of interest.
But from my experience, once a trader become experienced in watching this couple of times, he knows something is wrong and he can exit the long once selling starts. He may or may not take short but with experience, he knows it is better to exit long as selling is quite intense, fast and furious.