All this shit happened in almost the same fashion on 4chan several years ago. No, not 2012, or 2013- but much earlier. And not just on Holla Forums.
See, 4chan had been steadily going to shit (well, a steeper incline, anyway) around 2008-2009 (probably because of the scientology shit, lulzsec, GNAA, etc bringing unwanted attention). Various boards- the most obvious of which being Holla Forums- started to form noticeable shitposting cliques. Regular "meta" threads full of unwanted content, uncontained faggotry, avatarfagging, rampant tripfagging, etc. This obviously degraded the overall content of the board, and created new generations of dumb tripfags, shitposters, etc who just felt that was "the way of things". But it wasn't just contained on Holla Forums.
Instead, it spread to almost every board- even /x/- to the point that the board became almost impossible to post anything even tangentially related to the subject matter without said tripfags and circlejerks shitting up every thread. Which still wasn't anything new, but then these faggots started getting mod/janitor status. Suddenly, perfectly fine threads were getting deleted, while completely off topic and generally retarded threads were kept alive.
Why?
Ideological subversion on an "e-culture" level. Unlike offline "active measures", the demoralization stage doesn't take NEARLY as long. You don't need to wait for generations of children to be born during developmental phases. Imageboards, forums, etc all run on an almost purely cultural basis- and culture is made up of memes, fads, etc. These things are MUCH shorter lived- it's why it doesn't take too long to "get used to" a new workplace, or community, or what have you. Internet forums aren't deeply entrenched in identity, and anonymous imageboards especially aren't.
These are efforts designed not just to ruin the experience and discussion of its pre-established users, but to create an environment where new people will see a particular way of things. They see rampant shitposting and completely ridiculous threads as "the norm". Even long after the threads are gone, the idea remains.
Not when there's the same names, the same faces, the same stupid fucks always causing the same fucking problems. It's a concentrated effort by a small minority of individuals with unabridged power and freedom to exercise such. I don't care if they're "not kikes" or whatever non-argument is thrown about- it's very obviously a concentrated effort to shit up Holla Forums.
Regularly deleting threads or posts that fit the culture of Holla Forums, while regularly ignoring typical D&C threads, is not normal.