I am no expert myself but mass dumping often triggers automatic...

I am no expert myself but mass dumping often triggers automatic stop losses which people set based on a price trigger.
So someone with a bit of cash sets up a massive buy at a lower amount. They usually prey on fear and sentiment so when it goes tumbling down it tumbles fast. There are obviously many other techniques but don't have much idea about.

One thing I observed when the SP got smashed went something like this.
Look at the depth - the buy side was high, and so was the sell side. The 1M volume trades are set up on the buy side.
Someone with deep pockets sets this up. They (1 or more individuals) may just dump another 1M SELL just as the 1M+ buy gets removed. The price drops. The problem is that you can never tell if it is a genuine buyer or seller and the large volume may be split up into smaller amounts.

These guys are good!
That said sometimes they can also lose money if the market keeps buying.

Sometimes large investors may drive it down for a while to accumulate at a cheaper price. Hold it for a while and when the sentiment is right they sell.