In its IPO documents, Reddit said the price and volume of its stock could “experience extreme volatility for reasons unrelated to our underlying business.”
Considering they're DMing certain Reddit users (their most active users, according to the DM) about being given a chance buy Reddit stock during the IPO (that's not strictly PR speak; they're actually setting aside some stock for their users), that's entirely possible.
I think it's to limit damage from subreddits like WSB. If a bunch of your most active users own stock, they will be slandering WSB and calling for the ban of the subreddit when they eventually want to short it.
Reddit has enough of the same half crazed energy Tumblr has that I suspect quite a few users even the most active site would fuck the site over for fun.