It's true whenever bitcoin pumps altcoin normally dumps same thing with when bitcoin dumps, I think the real reason is that whenever bitcoin pumps alot of traders/investors don't want to miss out on it so they simply sell their altcoin holdings and buy bitcoin instead, while for the dumping part, I feel whenever bitcoin dumps alot of people(weak hands) become scared and panic sell their altcoin holdings, that's why people say altcoin follow the price movement of bitcoin.