Between USD and Bitcoin there is a relationship of value. Typically, Bitcoin is usually converted into USD. And most investors will use the dollar to buy Bitcoin speculation. So, if the dollar price drops, the value of Bitcoin will also decrease.
However, the rise and fall of the USD price did not significantly affect the growth or decrease of Bitcoin prices in the Cryptocurrency market.
There is no "if" dollar falls. Dollar will always fall slowly because of inflation in the world. I mean 100 years ago 10 dollars worth a lot of money and you could buy a lot of stuff with it. Nowadays 10 dollars is not enough for a decent meal let alone anything else. Dollar always goes down, all currencies goes down. This is the reason why people say that bitcoin is not a "currency" but more like gold and silver which gains value over the inflations of currencies like dollar and euro.
When dollar goes down bitcoin ends up higher no matter what happens because dollar values less. Maybe one day a mcdonalds burger will worth 100 dollars maybe that happens in like 20 years from now, at that point bitcoin will worth a million dollars and it wouldn't be a surprise or a shock because a million dollars won't be enough for a small house because dollar devalued. Bitcoin needs to go higher than inflation for it to be a good investment.