It was definitely part of an intentional, concerted effort to purge of the growing leftism on the site, with r/cth eing the most threatening example because it was the most popular. Like most social media platforms at the time, reddit was really starting to lock down what was ideologically acceptable. I've commented about it before, most recently here:

they banned r/cth because we kept posting John Brown memes

That may have been their excuse to pull the trigger but the reason they banned us was because the sub was becoming very well known, with posts regularly making it to the front page. During a time when reddit was really starting to actively sanitize itself of any actual leftism (for the sake of advertisers, profit, etc. as well as for the sake of becoming the major NATO propaganda hub that it is today) r/cth represented the complete opposite of the direction they were steering the platform. It was quite literally a purge of leftists across the platform and it would have happened whether or not we toed the line by not saying slave owners deserve death anymore or posting John Brown memes. In short, the sub was just starting to get some hints of more mainstream internet attention and there was no way in hell they were going to let that stand.

Edit: fixed the link. But also want to mention that Booty is totally correct about the_donald getting banned as cover, using the "both sides" justification when wiping out anything to the left of Liz Warren stans from the site.