For a long time, subreddits were not considered "communities". Redditors were one community and they used subreddits as sub forums. Communities were created through increased moderation which identified, grouped and isolated users. When communities were created, "Redditor" as a common shared identity was destroyed.

What happened was that around Trump running for office, and with an increase in users overall, certain subreddits (most of them not political btw) became more active and for want of a better word more "problematic" than others. The reddit admins, active mods and other vocal internet users wanted this content removed. To do this they added "quarantine" in an effort to restrict the activity of identified subreddits, and called these subreddits "communities" for the first time. Visiting a subreddit was labelled as joining a community.

By isolating subreddits they further isolated the users of reddit from each other, but they increased the group identity and feel of individual subreddits. Subreddit mods were very happy with this and other concerned users were happy with the politicisation via group creation of reddit.

A group identity becomes stronger the more one defines it in opposition with other groups.