A market crash can be a manipulation made by smart players to make money, taking advantage of small investors who panic and sell.
One of the biggest drivers of the Bitcoin price are low worldwide interest rates, leading individual investors to bet on Bitcoin.
i wouldn't necessarily call it the biggest driver, but i've always believed BTC and the crypto market has benefited from the "risk on" market cycle in the global economy, which has been characterized by easy credit. easy credit leaves investors scrambling for places to park money. most of it ends up in equities and real estate. some of it trickles into BTC, i'm sure.
if the stock market crashes, i don't think it will
cause BTC to crash. i don't think there is a causal relationship there. but if we see a 2008 redux, BTC will probably crash for the same reasons stocks will. money will flow out of all speculative assets and everything will drop in value across the board.