This is common on ICO that has a bounty for their support. It is because of many bounty hunters that dumps their tokens for their own profit. But not only that, that is not the only reason. 1 Reason again is also the project itself. If the project is not that good, it deserves to go down.
bounties are always blamed on this. well, but that has the truth, even though that makes dumps also sometimes from investors. however, selling after a list on an exchanger has become a common thing, and it usually happens. everyone's needs make that happen.