It is usually because of panic sellers and those ones who want to sell their airdrop or bounty tokens, preselling with bounos, airdrops and bounties are the reasons of dumps, that is why some very good ico never do that things, they gather fund with their abilities and their project not by advertising.
Does that prove that the role of bounty hunters who sell a few coins from their work can make prices go down so much? I think that is not something that will happen because the tokens they hold are certainly not comparable with tokens obtained by investors who bought them when the ICO took place, because every ICO purchase of course offers a big bonus so investors can get profits even though prices fall below ICO prices with the bonus they get right away.