Reddit Inc.'s stock-trading enthusiasts made it a household name by backing companies that Wall Street wouldn’t. For Reddit’s own initial public offering, a group of them are about to flip the script.

In the days after the social-media company filed for an IPO, thousands of members of the WallStreetBets forum — which boasts around 15 million users and helped popularize meme stocks like GameStop Corp. — voted to boost a forum post about shorting the company, a sharp reversal of their typically bullish ways. Their avowed reasons varied from the company’s lack of profitability to competitive concerns, and mostly centered on spite.