That's a good questions, and I sometimes wonder that. Why would someone say invest $1,000 into something, and then go on to sell it for $500. I'm guessing it's the market. They feel that there are better opportunity elsewhere.
For example the alt market it hurting, so it might be wiser to take your money out and put it in Bitcoin.
They are impatient buyer. They just want to get fast profit, don't need to know what is the value ICO project will bring back. But have someone would sell if they see the market is going down, and buy back at lower price to increảs the number of token they are holding.