It's the Danbooru upload bot. It's malfunctioning again. It happens every now and then. It's been happening a lot recently though, possibly due to Danbooru doing maintenance on its database.
I've been trying to figure out who runs the bot and how it works so we can at least know in advance when its going down and for how long. I still don't know much about it.
Until it starts working again, the only new posts that are on will be the ones users from this community will upload. If you look at our stats page, out of our top 10 uploaders, 98.3% of all of those posts come from other upload bots (Gelbooru, Nekobooru, Konachan, Moeimouto). And that's without factoring in danbooru's contribution of 315819 posts. That means that there's a very very small percentage of actual user uploaded content compared to bot uploaded content.