But we're talking about a substantial viewership drop, across a single platform (only desktop), all simultaneously on a single day. That's clearly not any sort of organic change.

Everything is driven by the algorithm. You can have big changes on one day and for specific platforms, even if it's something that's been building up gradually, because that might be the day the algorithm adjusted. It's hard to talk about "this caused that to happen" if you don't know what the algorithm is doing.