WHY WOULD ANYONE DO THAT?

For example, a whale may dump bitcoins on the market after going short on the futures market. Pushing prompt price down, and earning big on those shorts. And later, perhaps the opposite - going long in futures or options markets and buying bitcoins to push price up.
This is all possible with enough money but the market is very volatile itself. So it may go on rollercoaster also without conspiracy.

Bitcoin is important, but "altcoins" are more interesting for me, many players are invested there. I should use more blockchains, their services to see what is happening there. Going to try...