When bitcoin rises, the altcoins keep the price and if when the bitcoin falls the altcoins fall even more. Why do altcoins usually follow bitcoin?
There are two things to note here which will make you understand more on why altcoins follow bitcoin.
1. When bitcoin falls, the price of altcoins fall as well because people generally look at it's value in fiat (eg: USD price)
Since bitcoin's price falls in fiat the relevant price of altcoins fall as well.
2. When bitcoin falls, it creates a panic in the crypto market since it owns around 50% of the crypto market cap.
So this triggers a panic sell and includes altcoin holders along with it. Hence the price of altcoin even more.