Just happened minutes ago, so not much information yet.

I think it's likely that this article from Monday might have finally pushed it over the edge (since it's usually media attention that does it): You can’t offer to murder cops on Reddit unless you’re on r/TheDonald

The quarantine message says:

It is restricted due to significant issues with reporting and addressing violations of the Reddit Content Policy. Most recently the violations have included threats of violence against police and public officials.

As a visitor or member, you can help moderators maintain the community by reporting and downvoting rule-breaking content.

Here's the message the admins sent them:

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.

We hope that this process provides a viable way forward to restore the health of the community. However, if this situation continues to escalate, we will explore further actions, including the possible banning of your community.

Please confirm that you have received and understand this message.