If the stock market goes down, then investors would start searching for alternative (more profitable) investments and turn to Bitcoin and cryptocurrencies.
Crytocurrency investment is much more than an alternative but rather a big gamble to stock market investor. Most of them opt to invest in a much stable investment for they know how it is better to gain a little bit than to lose big time (Bitcoin's volatility). If stock market went down most of the trades will be disrupted not only cryptocurrency but global economy in general.