According to the post about this in SRD [0], this was because of repeated calls to violence (something that has been well-documented), and the final straw was repeated incitements to commit violence over the situation in Oregon.

The following is an excerpt of the message the Reddit admins sent the moderators of /r/The_Donald:

(Edit: I originally posted an excerpt of the message from the OP in the SRD post. I've since found the full message in a comment [1], so I'll be replacing it here with that version. Of note is that the admins have disabled /r/The_Donald's ability to use custom styling, as they abused CSS to hide the report button in order to prevent people from reporting violent content to them. The comment also mentions that one of the moderators of /r/The_Donald was stripped of most of his permissions as well.)

> Dear Mods,

> We want to let you know that your community has been quarantined, as outlined in Reddit’s Content Policy.

> The reason for the quarantine is that over the last few months we have observed repeated rule-breaking behavior in your community and an over-reliance on Reddit admins to manage users and remove posts that violate our content policy, including content that encourages or incites violence. Most recently, we have observed this behavior in the form of encouragement of violence towards police officers and public officials in Oregon. This is not only in violation of our site-wide policies, but also your own community rules (rule #9). You can find violating content that we removed in your mod logs.

> As we have discussed in the past, and as detailed in our content policy and moderator guidelines, we expect you to enforce against rule-breaking content. You’ve made progress over the last year, but we continue to observe and take action on a disproportionate amount of rule-breaking behavior in this community. We recognize that you do remove posts that are reported, but we are troubled that violent content more often goes unreported, and worse, is upvoted.

> User reports and downvotes are an essential way that Reddit functions to moderate content. Limiting or prohibiting them prevents you from moderating your community effectively. Because of this, we are disabling your custom styling in order to restore these essential functions.

> As stated in our Moderator Guidelines, our goal is to keep the platform alive and vibrant, as well as to ensure your community can reach people interested in it. Accordingly, here are the specific terms of the quarantine and the next steps we are asking from you as a mod team to resolve this situation.

> Quarantine terms:

> Visitors to this community will see a warning that requires users to explicitly opt-in to viewing it. This messaging reminds users of the importance of reporting rule-breaking content.

> Custom styling has been disabled to restore the report and downvote buttons.

> We hope both these changes will help improve the signal around rule-breaking content and improve your ability to effectively address it.

> Next steps:

> You unambiguously communicate to your subscribers that violent content is unacceptable.

> You communicate to your users that reporting is a core function of Reddit and is essential to maintaining the health and viability of the community.

> Following that, we will continue to monitor your community, specifically looking at report rate and for patterns of rule-violating content.

> Undertake any other actions you determine to reduce the amount of rule-violating content.

> Following these changes, we will consider an appeal to lift the quarantine, in line with the process outlined here.

[0] https://old.reddit.com/r/SubredditDrama/comments/c5safq/rthe...

[1] https://old.reddit.com/r/SubredditDrama/comments/c5safq/rthe...