I has nothing to do with altcoins, as alts normally do what bitcoin does. Times like this proves that there is only one big name in crypto currencies and that is bitcoin. When things go south for bitcoin, the alts just follow and take even a bigger hit.
As for your question, the short answer is because most people are selling and not buying or holding. No reason to sell the coins though, so I can't actually tell you why bitcoin is going down. The news in term of development are actually decent, with LN getting closer to a release, so I think this is just temporary.