Let me explain why there is linkage between assets.
For example, you see that BTC has fallen by 5% today, and the structure does not look good. You think it may fall again tomorrow or the day after tomorrow, but you do not have BTC in your hands. You look at the altcoins in your hands that have not fallen yet. What choice will you make? If you are not optimistic about the trend of BTC in the next few days, you will most likely sell the altcoins in your hands. If everyone thinks so, it will cause a collective sell-off of altcoins.
Therefore, if BTC falls, other altcoins will also fall, and there is a high probability that they will fall deeper, which has become a "market consensus". Then there will be traders who run this trading strategy or even automated trading players in the market, and further deepen this "market consensus" through trading.
In fact, it is best to use US stocks and BTC to explain "asset linkage". If you have observed the linkage between the two, you will find a particularly interesting phenomenon: "The decline of BTC is usually preceded by the decline of US stocks."
Why? Suppose I am an institutional fund manager. If I am not optimistic about the "future market", I must sell my "high-risk" assets first, so that my investment portfolio will be more defensive. (The "market consensus" here is: BTC is riskier than US stocks)
So when the market begins to be pessimistic about the future market development, BTC will fall before the US stock market. When the consensus of "the future market will be worse" is further deepened, everyone will start to sell US stocks, which will cause the US stock market to start to fall sharply.
