It is because most of the people need very fast money after investing, so they will start selling the coin once it is listed on the exchanges and due to bounty hunters, the price of a coin will decrease. The coin price will start increasing once the completely start following the developments along with the roadmap.
you are right, sometimes they sell bonuses, or all tokens they have when the token has been listed to a market. That is a natural thing, seeing some investors who want to use a good ICO to seek profits. however, I think that is a common thing, and if the developer is good, the price will recover.