The focus is on the Reddit army of small investors, but the explosion in passive investing has also played a big part in the GameStop market frenzy.

It's not just retail fueling the massive gains. Take industry giants such as BlackRock and Vanguard, who feature prominently among GameStop's top holders. Those firms have to hold shares of the company for their ETF products. As the shares gain, ETFs and other passive funds need to buy more stocks — these funds aren't able to sell the shares and take profits unlike stock pickers.