I was suggesting it had been incorrectly classified previously by a single vendor, presumably in response to a report (or a court order) for a single page. There's no nuance to the filtering, so the whole site gets blocked.
It happens regularly for Imgur: Someone posts CSAM, and the whole site gets added to Cleanfeed (the UK ISP level filter) for a couple of hours until the original image is removed.
I'm still seeking a source here.