randomposter

Going through this process but searching ALL worked great

in a way it does, when you're building massive scale systems. Say you are the mitigation team and want to protect yourself against a malicious hacker/employee that starts shutting down web servers or removes posting permissions from the DB for everyone. You're going to monitor the frequency of posts and if it drop too fast, you know something's bad happening. You're going to take automated measures against it - maybe freeze access to the DB completely, maybe switch to a (much less tested) backup region/system, etc.. so you can see how things can snowball from there to strange scenarios...

As an engineer, this sounds most plausible - they had proactive detection and resolution in place against various attacks and system failures, which got triggered due to the massive drop in public subreddits/users/activity, and made everything worse. Honestly, this isn't a scenario their engineers could have easily predicted...