There have been a few times where it has been demonstrated that whales have controlled the market, usually it is not intentional market manipulation, but just due to either a huge buy in, or sell out. For example there are a bunch of large known wallets which people track, if these wallets suddenly start moving large amounts of BTC around it is clear that the amount of BTC available increases, hence the average cost decreases.
Yeah, in a way whales always manipulate the market no matter what they do, if they try to sell their coins this is going to decrease the price of bitcoin not only because of the increase in the supply but because people see this as a sign that people are losing confidence in bitcoin, but if they decide to hold their coins they also have a huge effect on the market since that decreases the supply and makes the price to go up.