This may not be the reason for dumping because if we see the other side icos also create demand for btc and eth because everyone that will participate will have to buy and deposit btc and eth so that creates demand and i do not see serious projects sell all their btc and eth holdings quickly rather they exchange them as required.
If you compare good projects to scam projects, scam projects are outnumbering good projects and they always do exit scam after getting their fund, dumping their Eth and Bitcoin for Fiat.
But isn´t it good for the whole ecosystem? It helps the adoption because startups creating diffrent blockchain solutions and help the whole industry to grow.
Like apps on Android - more apps make Android more useful.
But some of the projects and concepts are just copy of past and existing projects, just take a look at the recent ICO's that uses an exchange model as their projects, so many of these ICO's are having a hard time launching their exchange, because they do not have something new and unique to offer, so they can't generate enough support.