i guess it because too many icos and if a year ago dump could be cause by bounty hunters, now its because of fake icos numbers, some ico could write on the site that it collected 10 millions usd for example and there is no single proof of it and they will make tokens worth 10 million usd and dump them
It's due to early investors and bounty hunters, a good investor or holder will support the project if it seems to have more stiffs to offer in the future that can make it pump.