The Chinese government banned ICOs. Naturally as a result, alt coins started dropping. People are fearful they won't be able to get their money out with the government cracking down.
For the next couple of days I think it will blow over and go back up, but only time will tell.
there is no valid reason that because of the banning the sudden drop happens. Chinese already did it a few months ago and still the crypto remains to sore high and higher than before.