People don't buy and sell ICO tokens with fiat, they do it with bitcoins. If a bunch of people want to sell their ICO tokens for fiat, they first end up with a bunch of bitcoins, and then they need to sell bitcoins for fiat. This pushes the price of bitcoins in fiat down.
It's a temporary phenomenon, but the price going down can make the price fall farther, because there are a bunch of amateur traders who think it's a good idea to buy when the price is rising and sell when it's falling.