However, I doubt any regulated banking institutions can do this legally! Shady OTC deals are happening for a long time so can't comment on that! But to bring down the price at this scale, the OTC volume needs to be enormous!
I think we should not be blinded by the pious image of the banks. Look what happened one week ago on the forex market...
"BlackRock and Allianz sue 16 Banks in the U.S. over Currency Market Rigging"
https://btcmanager.com/blackrock-and-allianz-sue-16-banks-in-the-u-s-over-currency-market-rigging/If they do it in the forex market, they will not be shy doing it in the Cryptos market.