Some people sell altcoins because they bought it with credit money and must pay. Most part of people are tier from present situation at market.
There are many reasons to sell altcoins soon. It is likely that investors are concerned that the exchange rate will fall further when the market becomes unstable. Also, if you invest in ICOs, early selling is a good idea because if you do not sell early, the exchange rate will drop drastically as other investors will sell out simultaneously when Token is listed on exchange floors.