Apart from the fact that Bitcoin is mostly quoted in the US dollar
It is the same with gold. Whenever the dollar starts to decline, the gold price starts to rise and vice versa. Since gold is basically forever it's value doesn't get much affected on its own, the change in its value is always caused by indirect factors. For example, the economy of the US starts to decline (or just slows down), people move their wealth into real assets such gold, and gold value rises. And this is apart from purely mechanical dependence between prices of gold and the dollar (as revealed by the USX index). Bitcoin is by far more more volatile on its own, of course, but its price is still negatively correlated with the US dollar mostly
The question is not about Bitcoin being different from the dollar
Indeed they are different. But that's not my point absolutely. The fact that they are not the same doesn't somehow belittle the
observation that their prices are correlated (negatively). And this is what we should expect since Bitcoin price is most often quoted in dollars. That's why the price of dollar directly and mechanistically affects Bitcoin. But this is not only a statistical relationship but rather a causal one. Moreover, it is unidirectional, i.e. it is Bitcoin that depends on the dollar strength (or rather weakness), but the reverse is not true. In other words, Bitcoin is not dollar's funeral