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!
That is why I think that while this story is probably true for some brokers, I don't think that banks could really get the majority to crush the market like this. I don't think they are smart and organized enough for these things. Such an elaborate plan needs massive cooperation to maximise the effect. Some banks can surely unite, but I don't think that enough of them can and care about cryptos at all. The reason behind the massive drop is BCH ABC and SV split. Bitcoin cash suffered severely because of this, other top coins also gave up their previous positions.