The only reason is what happening to bitcoin. Whenever bitcoin price falls, altcoins prices fall too and altcoins price fall is usually bigger than bitcoin.
I suppose many reasons cause this fall. Take at least the recent news about the ban of ICO ads by Google, Facebook's prohibitions, etc. You can find here more answers if you watch it:
https://www.youtube.com/watch?v=ZoAQl5V2m0s