When bitcoin starts a bull run, many of the altcoins fall, as investors jump on the Bitcoin train and vice versa. It’s also systemic because most exchanges require Bitcoin rather than fiat currency to transact. It is easy to invest fiat currency in the market and then leave there as an investor trades it; moving it from one currency to another and not cashing it back to fiat currency. Furthermore, When the Bitcoin price falls or rises against the fiat currency, all the altcoins will usually follow. This is reason why bitcoin can control the market.