All the big investors, hedge funds and investment firms use computers to track everything. They probably all use the same algorithms and those algorithms said, “sell”.
I don’t think it’s much more mysterious than that.
Algorithms looking for statistical arbitrage (as opposed to true arbitrage) often move together, even if the methodologies used are different.
Since these securities quickly recovered, it’s evident that other traders were looking for arbitrage opportunities on the opposite side of the trade.