that is because some altcoin makes bitcoin values as benchmarks for example ETH/BTC, XLM/BTC, BCH/BTC. So when the bitocin price drops it is a sign of decreasing demand and it will also cause the demand on altcoin to decline this according to market logic. Some argue it is due to coincidence only because in fact the cryptocurrency is self-contained, and I see when bitcoin price drops there are some altcoin even up.