Altcoin follows Bitcoin because of supply, demand and speculation. It should be understood that the inter coin has a relationship with one another not to mention the currency market. As a trader just follow & exit before the price reverses direction. Indeed if at a glance confusingly where prices suddenly rise or fall, but there is a chance to make a profit. Try searching again about what causes the down / up is anything. Then use that knowledge to trade (fundamentally). thanks.
This is all about the portion of the market Bitcoin has captured. The more portions a crypto has in the market, the more it has the ability to influence the market. No if we talk about Bitcoin, we already know it has captured a huge portion of the market and it can be thought of as monopoly which is actually the type of firm followed by the rest of the market. Since alt is also demanded a lot so when the price of Bitcoin go up, as a result of rise in the demand of alt, its price also go up.