Because people who participated in the Bounty Program that received the tokens and early investors who got discounted are selling(not all of course but majority of them) it right after they have it and when it's trad-able in exchange/s. People want valuable coins to sell as soon as possible from every ICOs before everybody else does(even myself too) and that's the main reason(for me). If an ICO has a good mission and vision then there is nothing to worry about the price of their token.

If you trust the Coins/Tokens then hold otherwise sell it, It's a matter of choice and we all want money(That's why we all doing Bounty).