people are dumping the alts to be able to get bitcoin forks i believe...
I forgot about all the forks that are coming out. Do you know if we get the splits from forks if we keep our coins on coinbase? I know for coins kept on blockchain, you can get the equivalent amount but I'm not so sure for coinbase.
As for the fall of altcoins, relax you'll be fine. Day trading is risky so just hold onto some good coins and watch the news.