Basic answer that you can get to understand this easily and it is because many are selling.
Why you would ask if you can sell your altcoins now? Are we the one that's holding your portfolio? If you are good with the current gains you have made then its better for you to sell. But don't ever do this trick, buy high - sell low. Those people who are in panic now are doing that trick and that should be avoided that's not the way you trade and make money in crypto.