When someone places a bet with borrowed money and the price moves hard against them, there comes a point where the exchange steps in and force-closes their position to stop the losses spiralling. That forced closing is called a liquidation.
The problem is that each of these forced sales shoves the price even further in the same direction — which trips the next person's liquidation, which shoves price further still. One after another they topple like a row of dominoes. That chain reaction is a cascade, and it can send price crashing (or spiking) shockingly far in just a few minutes.
As brutal as they look, cascades actually do something useful: they flush out all the over-eager, over-borrowed bets in one go. Once the dominoes have finished falling, the market is often far healthier and lighter — which is exactly why these violent flushes so frequently mark the precise turning point before a bounce.