Imagine 3 whales with 1000 btc each decide to sell bitcoin and by chance they do so at the same time. Of course the price will tank and people will wonder why, bots will follow and so on.
The possibility of whales selling or dumping 1000
BTC at a particular time in the open market is highly unlikely. There is an OTC market where big sales are taken place which is not reflected in the open market and we could expect these kind of whale manipulation when there is a rally and there is a big influence of institutional investors and they usually manipulate the market like you said.