I think it's almost entirely emergent based on how the incentives and pressures of self-selecting subreddit membership intersects with the upvote/downvote system.
An interesting and notable thing in this context is how for any given interest represented on reddit there is actually an entire ecosystem of subs. There's the main one, the decade-old mod schism one, the "circlejerk" parody one, the "True" one that takes itself more seriously, the more recent schism that allows slurs, the one for memes about the interest, a few to many for various subdivisions or specializations within the interest, various vestigial zombie schisms and misspellings that may or may not just redirect to one of the others.
Each of these will have elements of its own culture and history and jargon and in-jokes, but drawn from the same pool of subject matter and all in some way referencing or revolving around the biggest one, even if only in opposition to it. All will share some mods with at least some others and most users will frequent more than one of them.
What you end up coming up against is identity. Each sub needs a reason for itself to exist and so members create that meaning by enforcing the norms, referencing the jokes, and socializing new members into the history and protocol. HN works exactly like this too btw; it is culturally a subreddit, even if hosted elsewhere with slightly different game mechanics.