It is very easy to ban someone. Making the ban permanent and combining this with the moderator blocking the person (so they can't send messages), there's no appeal process.
Another part is that for any sub of reasonable volume, trying to actively moderate and shape beyond banning the most egregious actors is difficult. Deleting and locking posts for a finer level of moderation is time consuming. The judgement calls of "when is this going off the rails?" become more snap over time.
With the time consuming nature of actually moderating a sub and the ease of just banning someone - moderation becomes the policy of whoever has the most time. The stereotypical variations of this are the paid social media manager who's job it is to scrub anything positive of a competitor or negative about their brand, or a person who is moderating because of a deep interest in the subject but with strong opinions too.
With multiple active moderators, the most extreme views of each in turn become the overall "moderation philosophy" (and if those views are opposed the oldest one wins).
Combined with the echo chamber nature of the message board, the more and more extreme stances become the dominant stances.
To try to present a consistent approach to moderation (Reddit has gotten burned by inconsistent responses many times in the past) it appears that Reddit.inc is trying to be completely hands off. That in turn means that it takes extreme situations for corporate to get involved - often long after it's been a problem that they've been alerted to. Having let the problem fester for so long, when something is done, it tends to be very heavy handed, lopsided, and generates a significant amount of discontent that spreads elsewhere.
So, you've got a site that hosts thousands of message boards, that inevitably grow more and more partisan to one extreme or the other, are mostly facades for a corporation, or propaganda for a political organization.
It is impressive that it has remained "stable" for as long as it has.